fix(build): make --tag optional and repeatable, matching Docker

[BluPix]

Problem

mocker build declares -t/--tag as a single, required option. That breaks two everyday Docker usages:

$ mocker build .
Error: Missing expected argument '--tag <tag>'

$ mocker build -t app:1.0 -t app:latest .
...
Successfully tagged app:latest      # app:1.0 is never created

In Docker, -t is optional and can be repeated. container build supports both as well: it accepts repeated -t, and without -t it names the image after a random UUID. Only mocker's option declaration got in the way.

Change

• Build.tag is now [String], so -t can be omitted or repeated. The extra tags are passed through a new extraTags: parameter (default []) on ImageManager.build / makeBuildArguments as additional -t flags. Existing callers are unchanged.
• When no -t is given, mocker picks the same <uuid>:latest name that container build would use. Mocker chooses the name itself so it can still look up the built image and report its ID.
• Successfully tagged is printed once per tag given on the command line, and not at all for an untagged build, as in Docker.

Tests

• New unit tests:
  • CLI: --tag is optional and repeatable.
  • Argument builder: extra tags become additional -t flags.
• swift build and swift test pass.
• Tried by hand against container 1.5.0:
  • mocker build . builds and stores <uuid>:latest.
  • mocker build -t a:1 -t b:2 . creates both tags on the same digest.

Separately, mocker build -q hangs. That is upstream in container build -q (apple/container#2387, fix in apple/container#2388), not in this change.

🤖 Generated with Claude Code

[us]

hey @BluPix thanks for this great pr, will review soon and merge!