Building container for updated dependencies if not yet on development branch

[Zethson]

@steffenlem just ran into the issue where for his sync PR the used container still had the old MLflow version, but his new code required the new MLflow version. However, the PR does not build the container, only when it's merged.
You can of course build the container locally. The workflows don't care about this, because they rebuild the container locally anyways. Nevertheless, the workflows that do need the container would be faster if they could just pull the container and would not build it locally.

What is a good solution for this?

1. Build on PR? Might be too many builds and people might submit broken containers.
2. Build if the commit message contains some string? e.g. "force build container" or whatever. I think that this might be a nice solution, but must be documented.

I think that nf-core still just pushes a new container on pushes to dev?

CC @emiller88 @KevinMenden