The Docker images used for our continuous integration builds currently
live in the libgit2/libgit2-docker repository. To make any changes in
them, one has to make a PR there, get it reviewed, re-build the images
and publish them to Docker Hub. This process is slow and tedious, making
it harder than necessary to perform any updates to our Docker-based
build pipeline.

To fix this, we include all Dockerfiles used by Azure from the mentioned
repository and inline them into our own repo. Instead of having to
manually push them to the CI, it will now build the required containers
on each pull request, allowing much greater flexibility.