rebasing does not prune previous container image

[cgwalters]

Doing e.g.

$ rpm-ostree rebase ostree-unverified-registry:quay.io/fedora/fedora-coreos:stable
$ rpm-ostree rebase ostree-unverified-registry:quay.io/fedora/fedora-coreos:testing-devel

etc. will continually accumulate container images locally that won't be pruned. This is particularly problematic given e.g. the MCO will rebase by default.

Fixing this is a bit harder than I thought because it gets less into bugs but touches on some design issues in ostree-ext.

So far fixing this has needed:

• container: Prune image layers by default ostreedev/ostree-rs-ext#401
• Always use merge commit for container deployments #4137
• container: Add an API to query information from a commit object ostreedev/ostree-rs-ext#403
• container: Add deployed commits into set of GC roots ostreedev/ostree-rs-ext#404
• Properly prune previous containers when rebasing #4138