To Dockerize an app, you write a Dockerfile that describes how to build an image, build it with docker build, and run it with docker run while publishing the app’s port. Add Docker Compose only when you have run options or companion services (a database, cache or queue) worth recording. Docker’s documentation puts the split this way: “A Dockerfile provides instructions to build a container image while a Compose file defines your running containers.” (Docker Docs). This guide walks the path in order. The commands are an instructional workflow and were not run against your project, so adapt them to your language and framework.
Step 1: Inspect the app before writing anything
A Dockerfile is a written-down version of how your app already runs. Collect these facts first:
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- Language and runtime version (for example, the exact Node, Python, Java or Go version you develop against).
- Dependency manager and manifest files (
package.jsonand a lockfile,requirements.txt,pom.xml,go.mod, and so on). - Entry point: the exact command that starts the app.
- System packages needed, especially for native extensions.
- Listening port, and whether the app binds to
0.0.0.0rather than onlylocalhost. An app bound only to localhost inside a container is unreachable from the host. - Configuration inputs: environment variables, config files, secrets.
- External services: databases, caches, queues, object storage.
If you can, confirm the app runs correctly outside Docker first. Debugging an app bug and a container bug at the same time is slow. No single Dockerfile fits every framework, so treat the examples below as a pattern.
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Docker’s Writing a Dockerfile page covers the instruction set. The sketch below uses a Node.js web app purely as an illustration; swap the base image, install command and start command for your stack.
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FROM node:22
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
EXPOSE 3000
CMD ["node", "server.js"]What each line does:
FROMpicks the base image. Pin a specific version tag rather than relying on a floatinglatest.WORKDIRsets the directory for the following instructions and for the running container.COPY package*.json ./andRUN npm cibring in only the dependency manifests and install from them.COPY . .copies the rest of the source afterwards, so editing code does not invalidate the dependency layer.EXPOSEdocuments the container port. It does not publish it; you still map a host port at run time.CMDdefines the startup command. Prefer the exec (JSON array) form so the app receives stop signals directly.
Step 3: Build and run it
- From the project root (where the Dockerfile lives), build:
docker build -t my-app .The trailing dot is the build context. - Run it, mapping a host port to the container port:
docker run --rm -p 8080:3000 my-app - Open
http://localhost:8080. The format ishost:container, so the right-hand number must match the port your app listens on. - If it fails to start, read the output in the terminal, or run detached with
-d --name my-appand usedocker logs my-app. Check the container’s state withdocker ps -a.
Common first-run failures: a wrong start command or path, a missing system package, an app bound to localhost only, and a port mismatch between -p and the app.
Step 4: Improve the build
Docker’s building best practices and its hands-on image lab, which covers layers, cache ordering, .dockerignore, non-root users, multi-stage builds, base-image choice and build secrets, are the references for this stage.
Add a .dockerignore early
The whole build context is sent to the Docker daemon, and COPY . . will pull in anything in it. Docker’s Compose quickstart-style guidance specifically demonstrates excluding .env so sensitive values do not end up in an image layer. A starting point:
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.git
node_modules
.env
*.log
Dockerfile
.dockerignoreAdjust it to your stack: local dependency folders, build output, virtual environments and editor files typically belong here.
Order layers for cache reuse
Docker reuses cached layers until an instruction’s inputs change. Copy dependency manifests and install first, then copy source, as in the sample above. Day-to-day code edits then rebuild only the last layers.
Use a multi-stage build when build tools are not needed at runtime
Multi-stage builds let one stage compile or bundle the app and a later stage carry only what runs it. Docker says this can reduce image size and security exposure; how much depends on your app, so measure rather than assume.
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FROM node:22 AS build
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build
FROM node:22-slim
WORKDIR /app
COPY --from=build /app/dist ./dist
COPY package*.json ./
RUN npm ci --omit=dev
USER node
EXPOSE 3000
CMD ["node", "dist/server.js"]Here compilers and dev dependencies stay in the first stage, and the final image runs as the non-root node user. For compiled languages the final stage may be even smaller, but only if the binary and its libraries are compatible with the chosen base.
Choose a base image deliberately
| Choice | Good for | Watch for |
|---|---|---|
| Full language image | Easiest start; includes common build tools | Larger, more contents to maintain |
| Slim variant | Smaller runtime with common libraries | Native dependencies may need extra packages |
| Alpine-based variant | Very small footprint | Different C library and tooling can break native dependencies; not universally best |
Pick the smallest image your app and its native dependencies work on, then consider how you will debug it, since minimal images ship fewer diagnostic tools.
Keep secrets out of the image
Do not pass secrets through ordinary build arguments or commit them to the repository, because they can persist in image metadata or layers. Use Docker’s build-secret mechanism for build-time credentials, and a runtime secret facility appropriate to where you deploy.
Step 5: Decide whether you need Docker Compose
A single docker run is fine for one self-contained container. Compose earns its place when:
- the app needs a database, cache or queue alongside it;
- your run command has grown long (ports, env vars, volumes, networks) and you want it repeatable;
- teammates need one command to bring up the same environment.
Compose can also describe a single service just to preserve its options. The Compose build specification lets a service point at your Dockerfile, and the Compose quickstart shows the basic workflow. Example compose.yaml for an app plus a database:
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web:
build: .
ports:
- "8080:3000"
environment:
DATABASE_HOST: db
depends_on:
- db
db:
image: postgres:17
environment:
POSTGRES_PASSWORD_FILE: /run/secrets/db_password
volumes:
- db-data:/var/lib/postgresql/data
secrets:
- db_password
volumes:
db-data:
secrets:
db_password:
file: ./db_password.txtServices reach each other by service name, so the app connects to host db, not localhost. Start everything with docker compose up --build and stop it with docker compose down. Keep db_password.txt out of version control and out of the build context.
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Step 6: Plan for data and lifecycle
Data written only to a container’s writable layer disappears when the container is removed. Docker’s quickstart notes this explicitly. Stopping a container (docker stop) keeps that layer; removing it (docker rm, docker compose down) or recreating it after an image update does not.
| Approach | Survives container removal? | Use for |
|---|---|---|
| Writable layer only | No | Temporary files, caches you can regenerate |
| Named volume | Yes (until you delete the volume) | Database files, uploads on a single host |
| External data service | Yes, independent of the host | Data that must outlive the whole deployment |
Note that docker compose down -v deletes named volumes too, so use that flag deliberately.
Step 7: Review before production
Docker’s Use Compose in production guide describes applying production-specific configuration, typically with a second file merged over the base. Items to review:
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- Published ports: expose only what must be reachable; keep database ports off the host.
- Environment values and secrets: production credentials differ from development ones and should not live in the repository.
- Restart policy: for example
restart: alwaysso services return after a crash or reboot. - Logging and monitoring: decide where logs go and how you will notice failures.
- Updates: after changing code, rebuild and recreate only the affected service, e.g.
docker compose build webthendocker compose up --no-deps -d web.
A production override is applied like this:
docker compose -f compose.yaml -f compose.production.yaml up -dBe clear about scope: Compose on a single server is a reasonable deployment for many small apps, but it is not a high-availability, multi-node orchestrated platform. If you need failover or scaling across machines, that is a different tool decision.
Quick Recap
Final checklist
- App runs outside Docker; port, entry point and dependencies known.
- Dockerfile uses a pinned base image, installs dependencies before copying source, and runs as a non-root user.
.dockerignoreexcludes.git, local dependencies and.env.- Multi-stage build used if build tooling is not needed at runtime.
- Persistent data lives in a volume or external service.
- Compose added if there are companion services or a run command worth keeping; production overrides reviewed.
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