Docker Docker Dockerfile and Building Images Multistage Builds Separate Build and Runtime Dependencies

1. Core Command / Concept

Multi-stage builds separate build and runtime dependencies by using multiple Dockerfiles to create a final image. Use the --build-arg flag to pass arguments to the build stage.

2. Step-by-Step

1. Create a new directory for your project and navigate into it: mkdir multi-stage-build && cd multi-stage-build
2. Create a file named Dockerfile and add the following content: FROM python:3.9-slim AS build-stage
3. Add the following command to install dependencies in the build stage: RUN pip install -r requirements.txt
4. Add the following command to copy the source code into the build stage: COPY . /app
5. Add the following command to build the application in the build stage: RUN python -m build
6. Create a new file named Dockerfile.runtime and add the following content: FROM python:3.9-slim
7. Add the following command to copy the built application into the runtime stage: COPY --from=build-stage /app/dist /app
8. Add the following command to run the application in the runtime stage: CMD ["python", "app.py"]
9. Build the image using the multi-stage build: docker build -t myapp -f Dockerfile.runtime .
10. Run the image: docker run -p 8000:8000 myapp
11. ⚠️ This deletes all stopped containers: docker system prune -f

3. Real Commands with Examples

Example 1: Separate build and runtime dependencies

• Purpose: Build a Python application with separate build and runtime dependencies.
• Command: docker build -t myapp -f Dockerfile.runtime .
• Brief explanation of flags: -t sets the image name, -f specifies the Dockerfile to use.

Example 2: Use --build-arg to pass arguments to the build stage

• Purpose: Pass a build argument to the build stage.
• Command: docker build -t myapp -f Dockerfile.runtime --build-arg VCS_REF=$(git rev-parse --short HEAD) .
• Brief explanation of flags: --build-arg sets a build argument, VCS_REF is the build argument name.

Example 3: Use COPY --from to copy files from the build stage

• Purpose: Copy files from the build stage to the runtime stage.
• Command: docker build -t myapp -f Dockerfile.runtime .
• Brief explanation of flags: COPY --from copies files from the specified stage.

4. Common Errors

Error 1: COPY --from not found

• Error message or symptom: COPY --from not found
• Why it happens: The COPY --from flag is only available in Docker 17.05 and later.
• One-line fix: docker --version ? Verify with 'docker version'

Error 2: --build-arg not found

• Error message or symptom: --build-arg not found
• Why it happens: The --build-arg flag is only available in Docker 17.05 and later.
• One-line fix: docker --version ? Verify with 'docker version'

Error 3: FROM not found

• Error message or symptom: FROM not found
• Why it happens: The FROM instruction is missing in the Dockerfile.
• One-line fix: docker build -t myapp -f Dockerfile.runtime .

5. Quick Checks

Scenario 1: Check if the build stage is working

• Situation: You want to check if the build stage is working correctly.
• Command: docker run -it --rm myapp /bin/bash
• What a correct response looks like: The build stage should be able to run the application.

Scenario 2: Check if the runtime stage is working

• Situation: You want to check if the runtime stage is working correctly.
• Command: docker run -p 8000:8000 myapp
• What a correct response looks like: The runtime stage should be able to run the application.

Scenario 3: Check if the COPY --from command is working

• Situation: You want to check if the COPY --from command is working correctly.
• Command: docker run -it --rm myapp /bin/bash
• What a correct response looks like: The COPY --from command should be able to copy files from the build stage.

6. Last-Minute Reference

• --build-arg sets a build argument.
• COPY --from copies files from the specified stage.
• FROM specifies the base image.
• RUN executes a command in the build stage.
• CMD sets the default command to run in the runtime stage.
• docker build builds an image from a Dockerfile.
• docker run runs a container from an image.
• docker system prune removes all stopped containers.
• docker --version displays the Docker version.