Multiple FROMs - what it means

The first answer is too complex, historic, and uninformative for my tastes.

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It's actually rather simple. Docker provides for a functionality called multi-stage builds the basic idea here is to,

• Free you from having to manually remove what you don't want, by forcing you to allowlist what you do want,
• Free resources that would otherwise be taken up because of Docker's implementation.

Let's start with the first. Very often with something like Debian you'll see.

RUN apt-get update \ 
  && apt-get dist-upgrade \
  && apt-get install <whatever> \
  && apt-get clean

We can explain all of this in terms of the above. The above command is chained together so it represents a single change with no intermediate Images required. If it was written like this,

RUN apt-get update ;
RUN apt-get dist-upgrade;
RUN apt-get install <whatever>;
RUN apt-get clean;

It would result in 3 more temporary intermediate Images. Having it reduced to one image, there is one remaining problem: apt-get clean doesn't clean up artifacts used in the install. If a Debian maintainer includes in his install a script that modifies the system that modification will also be present in the final solution (see something like pepperflashplugin-nonfree for an example of that).

By using a multi-stage build you get all the benefits of a single changed action, but it will require you to manually allowlist and copy over files that were introduced in the temporary image using the COPY --from syntax documented here. Moreover, it's a great solution where there is no alternative (like an apt-get clean), and you would otherwise have lots of un-needed files in your final image.

See also

• Multi-stage builds
• COPY syntax

As of May 2017, multiple FROMs can be used in a single Dockerfile.
See "Builder pattern vs. Multi-stage builds in Docker" (by Alex Ellis) and PR 31257 by Tõnis Tiigi.

The general syntax involves adding FROM additional times within your Dockerfile - whichever is the last FROM statement is the final base image. To copy artifacts and outputs from intermediate images use COPY --from=<base_image_number>.

FROM golang:1.7.3 as builder
WORKDIR /go/src/github.com/alexellis/href-counter/
RUN go get -d -v golang.org/x/net/html  
COPY app.go    .
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .

FROM alpine:latest  
RUN apk --no-cache add ca-certificates
WORKDIR /root/
COPY --from=builder /go/src/github.com/alexellis/href-counter/app    .
CMD ["./app"]  

The result would be two images, one for building, one with just the resulting app (much, much smaller)

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE

multi               latest              bcbbf69a9b59        6 minutes ago       10.3MB  
golang              1.7.3               ef15416724f6        4 months ago        672MB  

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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.

Let me summarize my understanding of the question and the answer, hoping that it will be useful to others.

Question: Let’s say I have three images, apple, banana and orange. Can I have a Dockerfile that has FROM apple, FROM banana and FROM orange that will tell docker to magically merge all three applications into a single image (containing the three individual applications) which I could call smoothie?

Answer: No, you can't. If you do that, you will end up with four images, the three fruit images you pulled, plus the new image based on the last FROM image. If, for example, FROM orange was the last statement in the Dockerfile without anything added, the smoothie image would just be a clone of the orange image.

Why Are They Not Merged? I Really Want It

A typical docker image will contain almost everything the application needs to run (leaving out the kernel) which usually means that they’re built from a base image for their chosen operating system and a particular version or distribution.

Merging images successfully without considering all possible distributions, file systems, libraries and applications, is not something Docker, understandably, wants to do. Instead, developers are expected to embrace the microservice paradigm, running multiple containers that talk to each other as needed.

What’s the Alternative?

One possible use case for image merging would be to mix and match Linux distributions with our desired applications, for example, Ubuntu and Node.js. This is not the solution:

FROM ubuntu
FROM node

If we don’t want to stick with the Linux distribution chosen by our application image, we can start with our chosen distribution and use the package manager to install the applications instead, e.g.

FROM ubuntu
RUN apt-get update &&\
    apt-get install package1 &&\
    apt-get install package2

But you probably knew that already. Often times there isn’t a snap or package available in the chosen distribution, or it’s not the desired version, or it doesn't work well in a docker container out of the box, which was the motivation for wanting to use an image. I’m just confirming that, as far as I know, the only option is to do it the long way, if you really want to follow a monolithic approach.

In the case of Node.js for example, you might want to manually install the latest version, since apt provides an ancient one, and snap does not come with the Ubuntu image. For neo4j we might have to download the package and manually add it to the image, according to the documentation and the license.

One strategy, if size does not matter, is to start with the base image that would be hardest to install manually, and add the rest on top.

When To Use Multiple FROM Directives

There is also the option to use multiple FROM statements and manually copy stuff between build stages or into your final one. In other words, you can manually merge images, if you know what you're doing. As per the documentation:

Optionally a name can be given to a new build stage by adding AS name to the FROM instruction. The name can be used in subsequent FROM and COPY --from=<name> instructions to refer to the image built in this stage.

Personally, I’d only be comfortable using this merge approach with my own images or by following documentation from the application vendor, but it’s there if you need it or you're just feeling lucky.

A better application of this approach though, would be when we actually do want to use a temporary container from a different image, for building or doing something and discard it after copying the desired output.