`spack buildcache push --tag`: create container image with multiple roots (#41077)

This PR adds a flag `--tag/-t` to `buildcache push`, which you can use like

```
$ spack mirror add my-oci-registry oci://example.com/hello/world
$ spack -e my_env buildcache push --base-image ubuntu:22.04 --tag my_custom_tag my-oci-registry
```

and lets users ship a full, installed environment as a minimal container image where each image layer is one Spack package, on top of a base image of choice. The image can then be used as

```
$ docker run -it --rm example.com/hello/world:my_custom_tag
```

Apart from environments, users can also pick arbitrary installed spec from their database, for instance:

```
$ spack buildcache push --base-image ubuntu:22.04 --tag some_specs my-oci-registry gcc@12 cmake
$ docker run -it --rm example.com/hello/world:some_specs
```

It has many advantages over `spack containerize`:

1. No external tools required (`docker`, `buildah`, ...)
2. Creates images from locally installed Spack packages (No need to rebuild inside `docker build`, where troubleshooting build failures is notoriously hard)
3. No need for multistage builds (Spack just tarballs existing installations of runtime deps)
4. Reduced storage size / composability: when pushing multiple environments with common specs, container image layers are shared.
5. Automatic build cache: later `spack install` of the env elsewhere speeds up since the containerized environment is a build cache

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