Multiple FROM statements in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does using multiple FROM statements in a Dockerfile allow you to do?

It allows you to create multi-stage builds, where you can use different base images in stages and copy only needed artifacts to the final image. This helps keep the final image small and clean.

beginner

How do you name a build stage when using multiple FROM statements?

You add an alias after the FROM statement like this: FROM node:18 AS builder. This name can be used later to copy files from that stage.

beginner

Why is multi-stage build useful in Docker?

It helps reduce the final image size by excluding build tools and intermediate files. You only keep what is necessary to run the application.

intermediate

What command copies files from one build stage to another in a multi-stage Dockerfile?

The COPY --from=stage_name /source/path /destination/path command copies files from a named build stage to the current stage.

intermediate

Can you use multiple FROM statements without naming stages?

Yes, but without names you cannot selectively copy files between stages. Naming stages is best practice for clarity and control.

What is the main benefit of using multiple FROM statements in a Dockerfile?

A To run multiple containers at once

B To allow multiple users to build the image

C To speed up Docker daemon startup

D To create multi-stage builds that reduce final image size

Multiple FROM statements enable multi-stage builds, which help reduce the final image size by copying only necessary files.

How do you refer to a previous build stage when copying files in a multi-stage Dockerfile?

A Using the stage alias with COPY --from=alias

B Using the image tag with COPY --image=tag

C Using the container ID with COPY --id=container

D Using the FROM statement again

You use the stage alias defined after FROM with COPY --from=alias to copy files from that stage.

Which of these is a correct syntax for naming a build stage?

A FROM python:3.12 LABEL builder

B FROM python:3.12 AS builder

C FROM python:3.12 STAGE builder

D FROM python:3.12 NAME builder

The correct syntax to name a build stage is 'FROM image AS name'.

What happens if you do not use multiple FROM statements in a Dockerfile?

A You cannot create multi-stage builds and the image may be larger

B Docker will automatically create multiple stages

C The build will fail

D You can still copy files from other images

Without multiple FROM statements, you cannot create multi-stage builds, so the image may include unnecessary build files.

Which command copies files from a previous build stage named 'builder'?

A COPY --image=builder /app/build /app

B COPY --stage=builder /app/build /app

C COPY --from=builder /app/build /app

D COPY --source=builder /app/build /app

The correct command to copy files from a named stage is COPY --from=builder.

Explain how multiple FROM statements help optimize Docker images.

Think about how you can separate building and running steps.

Describe the syntax and purpose of naming a build stage in a Dockerfile.

Naming helps copy files from one stage to another.

Practice

1. What is the main purpose of using multiple FROM statements in a Dockerfile?

easy

A. To create multi-stage builds that reduce final image size

B. To run multiple containers at the same time

C. To specify different base images for different containers

D. To define environment variables for the container

Solution

1. Step 1: Understand the role of FROM in Dockerfile

   The FROM statement sets the base image for building a Docker image.

2. Step 2: Recognize multi-stage build benefits

   Using multiple FROM statements allows building in stages, copying only needed files to the final image, reducing size.

3. Final Answer:

   To create multi-stage builds that reduce final image size -> Option A

4. Quick Check:

   Multiple FROM = multi-stage build [OK]

Hint: Multiple FROM means multi-stage build for smaller images [OK]

Common Mistakes:

• Thinking multiple FROM runs multiple containers
• Confusing FROM with environment variable setting
• Assuming FROM defines runtime commands

2. Which of the following is the correct syntax to copy files from a previous build stage named builder?

easy

A. COPY --stage=builder /app/build /app

B. COPY --from=builder /app/build /app

C. COPY --source=builder /app/build /app

D. COPY --stage-name=builder /app/build /app

Solution

1. Step 1: Recall the correct COPY syntax for multi-stage

   The correct flag to copy from another stage is --from=stage_name.

2. Step 2: Match the option with correct syntax

   Only COPY --from=builder /app/build /app uses --from=builder, which is valid syntax.

3. Final Answer:

   COPY --from=builder /app/build /app -> Option B

4. Quick Check:

   Copy from stage uses --from [OK]

Hint: Use COPY --from=stage_name to copy files between stages [OK]

Common Mistakes:

• Using --stage or --source instead of --from
• Misspelling the flag name
• Omitting the equals sign (=) after --from

3. Given this Dockerfile snippet, what will be the final image size behavior?

FROM node:18 AS builder
RUN npm install && npm run build
FROM nginx:alpine
COPY --from=builder /app/build /usr/share/nginx/html

medium

A. Final image includes full Node.js and npm layers, making it large

B. Final image will fail to build due to missing /app/build directory

C. Final image only contains nginx and the built files, keeping it small

D. Final image contains both Node.js and nginx running simultaneously

Solution

1. Step 1: Analyze the multi-stage build steps

   The first stage builds the app with Node.js. The second stage uses nginx base image.

2. Step 2: Understand the COPY --from usage

   The built files from the builder stage are copied into nginx image, but Node.js layers are not included.

3. Final Answer:

   Final image only contains nginx and the built files, keeping it small -> Option C

4. Quick Check:

   Multi-stage copy keeps final image small [OK]

Hint: Final image excludes build tools if copied from previous stage [OK]

Common Mistakes:

• Assuming all build layers stay in final image
• Thinking COPY --from copies the entire image
• Confusing build failure with missing files

4. Identify the error in this Dockerfile snippet using multiple FROM statements:

FROM python:3.12 AS builder
RUN pip install -r requirements.txt
FROM alpine
COPY --from=build /app /app

medium

A. The stage name 'build' does not match the defined stage 'builder'

B. Alpine image cannot be used after python image

C. COPY command syntax is incorrect; missing destination path

D. pip install command should be in the second stage

Solution

1. Step 1: Check stage names in FROM statements

   The first stage is named 'builder', but COPY uses 'build' which is undefined.

2. Step 2: Understand COPY --from requires exact stage name

   Stage names must match exactly; otherwise, Docker throws an error.

3. Final Answer:

   The stage name 'build' does not match the defined stage 'builder' -> Option A

4. Quick Check:

   Stage names must match exactly in COPY --from [OK]

Hint: Match stage names exactly in COPY --from [OK]

Common Mistakes:

• Assuming similar names are accepted
• Thinking base image order matters here
• Ignoring COPY destination path correctness

5. You want to build a Docker image that compiles a Go application and then creates a minimal final image with only the compiled binary. Which Dockerfile snippet correctly uses multiple FROM statements to achieve this?

hard

A. FROM golang:1.20 AS builder
WORKDIR /app
COPY . .
RUN go build -o myapp
FROM scratch
COPY --from=builder /app/myapp /myapp
CMD ["/myapp"]

B. FROM golang:1.20
WORKDIR /app
COPY . .
RUN go build -o myapp
FROM alpine
COPY /app/myapp /myapp
CMD ["/myapp"]

C. FROM alpine AS builder
RUN apk add --no-cache go
WORKDIR /app
COPY . .
RUN go build -o myapp
FROM alpine
COPY --from=builder /app/myapp /myapp
CMD ["/myapp"]

D. FROM golang:1.20 AS builder
WORKDIR /app
COPY . .
RUN go build -o myapp
FROM alpine
COPY --from=builder /app/myapp /myapp
CMD ["/myapp"]

Solution

1. Step 1: Identify the build stage with Go compiler

   Stage named 'builder' uses golang image to compile the app.

2. Step 2: Use a minimal runtime image for final stage

   The final stage uses alpine (small image) and copies only the binary from builder.

3. Step 3: Verify COPY --from syntax and CMD

   COPY uses --from=builder to get binary; CMD runs the binary correctly.

4. Final Answer:

   FROM golang:1.20 AS builder WORKDIR /app COPY . . RUN go build -o myapp FROM alpine COPY --from=builder /app/myapp /myapp CMD ["/myapp"] -> Option D

5. Quick Check:

   Multi-stage build with builder and minimal final image [OK]

Hint: Use builder stage for compile, minimal image for final [OK]

Common Mistakes:

• Using scratch without proper setup
• Copying files without --from flag
• Building in final stage instead of builder