Provide option to not install NCS

[matteoscordino]

Applications using a West T2 project topology (a.k.a. "Workflow 4" according to Nordic docs) would be downloading their own NCS repo with west in a typical CI/CD situation.

Specifically to our case, we use GitLab CI and we get the manifest repo downloaded before the CI script starts running, so the pre-existing NCS is both a waste of space and an inconvenience.
Sure, we could use the pre-installed NCS if we made sure to pull the docker image that matches the NCS version referenced in our manifest, but that defeats the some of the benefits of the T2 topology, e.g.:

• it means that we can't override repos from the NCS manifest to bring in custom changes from our own forks
• it means that we can't update our project to a newer version of the NCS until this repo is updated to it and a new image generated

Our first approach was to start our CI script with:

• removing the NCS west workspace
• west init -l in the manifest repo that is being built
• west update --narrow -o=--depth=1
• redefine ZEPHYR_BASE
• start the west build

This works, but it's wasteful of space and slow.

Our second attempt was to fork this repo to remove the NCS installation from the Dockerfile (lines 129:155), then in the CI script:

• pull the new "NCS-less" docker image
• west init -l in the manifest repo that is being built
• west update --narrow -o=--depth=1, which gets the NCS and various repos from the manifest on disk
• install the python requirements from the zephyr, nrf and bootloader directories
• start the west build

This means that the project is built with the NCS and all other dependencies as defined in the project's manifest.
The drawback is that the python requirements need to be installed as part of the build, which takes some time (but can be cached at the CI level)

In short:

• is there a better way (that I couldn't figure out) to use this docker image with a T2 topology?
• if the answer is "no, there isn't", I offer to create a PR that makes the installation of NCS conditional to a Dockerfile ARG (I propose the installation is on by default, and the ARG allows skipping it) and explains in README.md how to use this feature. I would like to first check that this approach makes sense, though =)

[coderbyheart]

1. Converted from issue to discussion because I think it's out of scope for this project.
2. I think what your are looking for is demonstrated in https://github.com/nrfconnect/ncs-example-application

[coderbyheart]

More specifically:

• is there a better way (that I couldn't figure out) to use this docker image with a T2 topology?

I've added #48, because I kinda think that it should be possible to use it for a T2 topology project, and it should provide a significant speed improvement over using the Zephyr Docker image.

• if the answer is "no, there isn't", I offer to create a PR that makes the installation of NCS conditional to a Dockerfile

but here I have to say that this is out of scope for this project. Because the purpose is to show how to build "nRF Connect SDK applications", and not arbitrary NCS projects, given that this is still an unofficial, unsupported, toy project.

[matteoscordino]

makes sense. I actually was assuming that ncs-example-application was using this docker image to build, but I now see that it uses Zephyr's.
I will test with that a bit and compare whether it's more efficient to use Zephyr's image or to keep using our fork of your repo.

Thank you!