fix(installer): reject invalid docker image references for --output early

[chrisdebian]

What

create-repo --type docker currently accepts whatever value is passed
via --output and uses it directly as a Docker image reference,
without validating it first.

Why this breaks (#360)

The generic create-repo example in the docs uses a filesystem path
for --output:

luet create-repo --name "test" --output $PWD/out --packages $PWD/out --tree $PWD/package

That's correct for the disk/http repository types. But the docs
also explain, a little further down, that the docker repository type
needs --output to be an actual docker image reference:

When specifying the docker repository type, luet will generate
final images from the build results and upload them to the docker
reference specified with --output.
(docs/content/en/docs/Concepts/Overview/repositories.md:118)

It's easy to miss that distinction and combine the generic example
with --type docker, which is exactly what #360 reports. The raw path
ends up embedded as an image tag in pushFileFromArtifact/
GenerateFinalImage, and gets rejected by Docker's own (stricter)
reference grammar deep inside a shelled-out docker load call — one
artifact at a time, with an opaque invalid reference format error
and no indication of what's actually wrong.

Fix

Validate --output as a real docker reference in
LuetSystemRepository.getGenerator, before any file walking or image
building starts, using github.com/distribution/reference (already
an indirect dependency of this module — promoted to direct here, no
new dependency added). On failure, the error now names the actual
requirement instead of surfacing a bare "invalid reference format"
from three layers down.

Testing

• Added a regression test (Docker repository validation context in
  pkg/installer/repository_test.go) reproducing the exact broken
  input from Can't create repository following documentation example with (--type docker) #360. It needs no Docker daemon, since the new validation
  runs before any Docker interaction.
• go build ./..., go vet ./..., and gofmt -l are clean.
• go.mod/go.sum diff is a single dependency promoted from indirect
  to direct — no new module added.
• Confirmed the new spec passes; the pre-existing Lchown-related
  test failures in pkg/installer reproduce identically on unmodified
  master in this sandbox (no root/CAP_CHOWN) and are unrelated to
  this change.

Fixes #360