Use images from Quay

[benz0li]

• Close https://github.com/commercialhaskell/stack/issues/6475

• [x] Any changes that could be relevant to users have been recorded in ChangeLog.md.

• [x] The documentation has been updated, if necessary

No updates to ChangeLog.md and documentation required.

[benz0li]

I now mirror the GHC musl images to both Docker Hub and Quay:

• https://hub.docker.com/r/benz0li/ghc-musl
• https://quay.io/repository/benz0li/ghc-musl

Docker Hub also imposes a rate limit. Quay, on the other hand, does not.

This also ensures the long-term availability of these images.

[mpilgrem]

Many thanks!