Introduce explicit image versioning

Over the past five years we have accumulated about 20 TB of Docker images; this is clearly unhelpful and unsustainable. The reason for this is that:

1. we tag all images only with their git commit
2. we have no means of identifying which commits are referred to by downstream users (principally ghc/ghc>)

To avoid this in the future we will need to be more explicit about versioning. I suggest the following:

• continue to tag all images with their git commit
• automatically GC these tags after 5 days
• also manually tag images which we use from ghc/ghc> or another downstream with an explicit version identifier
• only refer to images from downstream via the explicit version

This allow automatic GC of most tags, while making manual GC of explicit version tags feasible and preserving the ability to easily transiently refer to commits.