feat: make flyte build the supported CI image build path

[jeevb]

Summary

• Add tag: Optional[str] = None to Image.clone(), from_dockerfile(), from_debian_base(), and from_uv_script() — placed immediately after name in every signature, consistent with registry and name
• Replace the private Image._tag field with a public Image.tag attribute, consistent with registry and name; constructors now pass tag= directly to _new() instead of using post-construction object.__setattr__
• Thread force: bool = False through build_images → _build_images → _build_image_bg → build.aio so the existing ImageBuildEngine.build(force=) param is reachable from the public API
• Add --force flag to flyte build CLI via BuildArguments, wired into BuildEnvCommand.invoke
• Deprecate examples/image/ci_build_image.py with a comment pointing to flyte build

Usage

Pattern A — retag + rebuild via remote builder:

# images.py
env = flyte.Environment(
    name="my_image",
    image=flyte.Image.from_base("ghcr.io/flyteorg/flyte:py3.12-v2.0.0b56").clone(
        registry="123456789.dkr.ecr.us-west-2.amazonaws.com/myorg",
        name="my-image",
        tag=os.environ.get("VERSION"),
    ),
)

Pattern B — Dockerfile-based build:

env = flyte.Environment(
    name="my_image",
    image=flyte.Image.from_dockerfile(
        Path(__file__).parent / "Dockerfile",
        registry="123456789.dkr.ecr.us-west-2.amazonaws.com/myorg",
        name="my-image",
        tag=os.environ.get("VERSION"),
    ),
)

# CI step — always rebuild and push
flyte build images.py my_image --force

Test Plan

• uv run pytest tests/flyte/test_image.py tests/flyte/test_deploy.py tests/cli/test_build.py -v — 70 tests, all pass
• uv run flyte build --help — --force appears in output

[jeevb]

Thinking we should support bare images, not wrapped in Environment as well. Environment feels extraneous for this purpose.