Fix --tag parameter to properly push tags and update latest

[the-braveknight]

Fix --tag parameter to properly push tags and update latest

Motivation

The --tag parameter was accepted by containertool but had no effect on the
actual tags pushed to the registry. When running:

swift package build-container-image --repository ghcr.io/org/image --tag v1.0.0

The image was only pushed to ghcr.io/org/image:latest (the default reference
from --repository), while the output misleadingly reported ghcr.io/org/image:v1.0.0.

This made it impossible to publish versioned releases using the --tag flag.

Modifications

• Collect all tags to publish: the --tag value (if provided) plus the default
  reference from --repository (typically 'latest')
• Push the manifest and index to each tag in the collection
• Skip duplicate pushes when --tag matches the repository reference
• Return the --tag reference (if provided) as the primary result

Result

When --tag is provided, the image is pushed to both the specified tag AND the
default reference:

--tag v1.0.0 → pushes to v1.0.0 and latest
--tag latest → pushes to latest (once, no duplicate)
(no --tag) → pushes to latest (existing behavior)

Test Plan

1. Build with --tag flag: swift package build-container-image --repository ghcr.io/org/image --tag v1.0.0
2. Verify both tags exist: docker pull ghcr.io/org/image:v1.0.0 and docker pull ghcr.io/org/image:latest
3. Build with --tag latest: verify no errors and single push
4. Build without --tag: verify existing behavior unchanged