Multiple FROMs - what it means Ask Question

what is a base image?

A set of files, plus EXPOSE'd ports, ENTRYPOINT and CMD.
You can add files and build a new image based on that base image, with a new Dockerfile starting with a FROM directive: the image mentioned after FROM is "the base image" for your new image.

does it mean that if I declare neo4j/neo4j in a FROM directive, that when my image is run the neo database will automatically run and be available within the container on port 7474?

Only if you don't overwrite CMD and ENTRYPOINT.
But the image in itself is enough: you would use a FROM neo4j/neo4j if you had to add files related to neo4j for your particular usage of neo4j.

2018: With the introduction of the --target option in docker build, you gain even more control over multi-stage builds.
This feature enables you to select which FROM statement in your Dockerfile you wish to build, allowing for more modular and efficient Docker images. This is especially useful in scenarios where you might want to:

1. Build Only the Dependencies: Create an image that only contains the dependencies of your project. This can be useful for caching purposes or for environments where you only need to run tests or static analysis tools.

2. Separate Build and Runtime Environments: Compile or build your application in a full-featured build environment but create a smaller, more secure image for deployment that only includes the runtime environment and the compiled application.

3. Create Images for Different Environments: Have different stages for development, testing, and production environments, each tailored with the specific tools and configurations needed for those environments.

Example Using --target

Given a Dockerfile with multiple stages named builder, tester, and deployer, you can build up to the tester stage using the --target option like so:

docker build --target tester -t myapp-test .

This command tells Docker to stop building after the tester stage has been completed, thus creating an image that includes everything from the base image up to the tester stage, but excluding anything from deployer stage and beyond.

Dockerfile Example with --target Usage

# Builder stage
FROM golang:1.7.3 as builder
WORKDIR /go/src/github.com/example/project/
# Assume app.go exists and has a function
COPY app.go .
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .

# Tester stage
FROM builder as tester
COPY . .
RUN go test ./...

# Deployer stage
FROM alpine:latest as deployer
COPY --from=builder /go/src/github.com/example/project/app /app
CMD ["/app"]