Instead of just blindly accepting any characters to make up the docker
registry domain, reject invalid domains (including ones which contain
e.g. '@', '#', and '?').
flatpak_docker_reference_parse() asserts that the regex always matches,
but inputs containing newlines cause the match to fail since the regex
is not compiled with G_REGEX_DOTALL. Return an error instead.
Which allows one to create an image source from a container location.
It also adds a new FlatpakDockerReference to access different parts of a
docker reference and changes to FlatpakOciIndex to get a manifest for a
specific architecture.
This will become useful in the next commit when we're going to add
support for installing OCI images.
Sebastian Wick