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 microservices 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.
Example
I wanted a lean image with gpgv only, and based on this Unix & Linux answer, I installed the whole gpg with yum and then copied only the binaries required, to the final image:
FROM docker.io/photon:latest AS builder
RUN yum install gnupg -y
FROM docker.io/photon:latest
COPY --from=builder /usr/bin/gpgv /usr/bin/
COPY --from=builder /usr/lib/libgcrypt.so.20 /usr/lib/libgpg-error.so.0 /usr/lib/
The rest of the Dockerfile continues as usual.
