Add CD for nightly docker images

[Trolldemorted]

Nightly images that are automatically pushed to ghcr

Things you have to do yourself (once):

• decide which branch is the master branch (if sail2 is the master branch, replace master with sail2)
• create a PAT which can push public and private images
• put that PAT into a github actions secret called CR_PAT
• replace USERNAME with the PAT's owner
• ensure that users has the permission to push to rems-project
• set the image visibility to public once the first version is published (you must login as the PAT's owner, being an owner of rems-project does not suffice)

Fixes #107

[Alasdair]

Thanks for setting this up. I've never really used dockers/containers so I will have to do a bit of studying I think

[Trolldemorted]

No problem, that's more or less just copypasta from another project I migrated to ghcr. If you need docker-related help just ask, banishing the arcane ocaml incantations into containers should help users a lot and it solves the "works on my machine" problem by shipping that machine