Reduce size of opam-repository git clone

[talex5]

At the moment, we copy in the cluster worker's clone of opam-repository, which includes all branches and PRs.

We should instead just clone the master branch directly within the Dockerfile.

[lindig]

This might be interesting: https://medium.com/pinterest-engineering/how-a-one-line-change-decreased-our-build-times-by-99-b98453265370

[avsm]

Not an entirely trivial thing to fix, since following the COPY with a git branch -D will create another layer.