Simpler base image build: use docker build / remove ssh secrets during base image build

[thomas-riccardi]

Currently the base image build is overly complicated to be able to fetch private remote resources via ssh (probably only git+ssh), to handle dependencies on other private projects. This cannot be done using docker build without leaking the secrets in the docker image (may be doable with image squashing..), so current dmake does docker run + docker exec + docker commit.

This is overly complicated, we lose docker build caching, and it's not even needed: we should use submodules to include other private repositories (in practice the don't change that much).

[thomas-riccardi]

Related to #111

[thomas-riccardi]

197 relies on docker run+commit instead of docker build to work around wrongly configured machines having nvidia as default runtime engine.

Will need to stop doing that/dmake checks that?

[thomas-riccardi]

The base image could simply be a different target stage in the same Dockerfile (or a different Dockerfile?)

[thomas-riccardi]

This is blocking non RUN commands in base image: ENV for example.

[thomas-riccardi]

See also #505