lint: Add --fix

I really wanted to avoid bootc being involved in being a "build system" - we should clearly support multiple build systems.

Right now we have `bootc container lint` which is an optional thing to invoke as part of a build, and it has grown more and more checks. It's readonly today.

In this issue I propose adding `--fix` (much like `cargo clippy` and `cargo clippy --fix` etc.).

Some things like `/var/run` we could easily apply the fix for (delete the link and let it be created by systemd-tmpfiles) e.g.

## Fixing users and tmpfiles

In https://github.com/coreos/rpm-ostree/issues/5230 I was thinking about trying to split out rpm-ostree's code for dealing with `/var` -> tmpfiles.d into something that can be invoked in a bit more standalone fashion.

However if we have `--fix` here...I think we could consider moving the tmpfiles.d and sysusers.d handling into this project instead. It's clearly within our dependency set already.

Does anyone have opinions on this?

If we did things this way then what would happen is all the tmpfiles.d code would move into this project, and rpm-ostree would just call it.

Start with the `bootc container lint` command and review the issue discussion, then read the linked rpm-ostree issue about `/var` handling. The proposal is to add `--fix`, but the scope—individual fixes versus moving tmpfiles.d and sysusers.d handling into bootc—needs resolution before completion can be defined.

Vom Indexierungsmodell aus dem Issue-Text verfasst.