Move repeated steps of docker.yml to a called workflow

[infotroph]

          Each image's job has steps that are identical or close to it, correct? If so, I think we could likely save a fair amount of duplication by encapsulating these steps in a called workflow (see e.g. how check.yml/test.yml/sipnet.yml are used by ci.yml and ci-weekly.yml). 

(Probably just as easily done in a separate PR, so no need to hold this one up)

Originally posted by @infotroph in #3354 (comment)