Heptapod CI: using our own registry to avoid Docker Hub rate limits

This is a quickfix, we'll probably rehost the image in
`heptapod/ci-images` at some point.
3 jobs for branch/default in 47 seconds
Status Job ID Name Coverage
  Build
passed #121037
html-build

00:00:23

 
  Deploy
passed #121039
deploy

00:00:24

passed #121038
deploy-test

00:00:22