install: Make --skip-fetch-check more discoverable

[stefwalter]

When trying to use bootc install I cannot install from a local tag:

# sudo podman run --rm --privileged --pid=host --security-opt label=type:unconfined_t -v /:/target -v /var/lib/containers:/var/lib/containers quay.io/swalter/monday:4.0 bootc install to-filesystem --karg=console=ttyS0,115200n8 --replace=alongside /target
ERROR Installing to filesystem: Verifying fetch: Creating importer: Failed to invoke skopeo proxy method OpenImage: remote error: reading manifest 4.0 in quay.io/swalter/monday: manifest unknown

[cgwalters]

Per #436 (comment) you definitely can, just need to --skip-fetch-check. I'll expand the error message.

(This is what https://gitlab.com/bootc-org/podman-bootc-cli always does right now for example)

[stefwalter]

Which is this an error? Why isn't it a warning, by default?

[cgwalters]

Why isn't it a warning, by default?

The original intent of the fetch check (see --help for it) was around the registry credentials; it happened to pick up the local-only image case as well.

I am a bit uncertain about the visibility if we lowered this to a warning...but maybe what would help is to have a nice "warnings" section both during and after the install process is complete to ensure they're visible. (This also relates a lot to #381 ...in theory, we could also do the fetch check there as part of the build!)

Now regarding local images, I guess there's at least four cases:

• Installing from a registry, things are configured (the "default")
• Installing from a registry tag, but I'm working offline e.g. ✈️ and I don't want to go check the internet (same rationale behind why podman run has e.g. --pull=newer that's not on by default)
• Installing from the special localhost/ podman registry tag that cannot be remote, e.g. localhost/someimage...we should probably special case this
• Installing with --target-imgref being a non-registry transport like oci-archive (this one it seems like we should probably just skip the fetch check, as you may be intending to update over e.g. USB keys which aren't plugged in)

[cgwalters]

We ended up implementing this in #1256