test(e2e): handle registry ports in combined image names

[myetcd]

Description

Closes: #2701

Extract source image tags after the last slash when naming combined provider/consumer images. The existing strings.Split(image, ":")[1] expression turns localhost:5000/org/repo:v1 into 5000/org/repo, resulting in an incorrect combined image name.

This is defensive hardening of the E2E naming logic: current source-image builders use hard-coded GHCR or local names, and --docker-image bypasses the combined build. This change does not introduce custom-registry configuration.

tests/e2e/docker_image.go contains the naming helper, and tests/e2e/test_target.go propagates its errors. Explicit tags are required; missing tags and digest references return errors. Regression tests cover local/GHCR names, registry ports on both images, mixed sources, local build tags, IPv6 registries, and unsupported references. The E2E README documents the naming contract.

Validation:

• The original naming expression fails the registry-port, mixed-source, and IPv6 regression cases.
• go test ./tests/e2e -run '^TestCombinedDockerImageName' -count=1 passed.
• go test ./tests/e2e -skip '^TestWriteExamples$' -count=1 passed (the skipped test generates example files).
• git diff --check passed.
• make lint passed (0 issues).
• make build passed.
• make test-unit passed.
• Docker-backed E2E tests have not been run: the local Docker daemon is unavailable.

---

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• Included the correct type prefix in the PR title
• Targeted the correct branch (see PR Targeting)
• Provided a link to the relevant issue or specification
• Reviewed "Files changed" and left comments if necessary (no inline comments needed)
• Confirmed all CI checks have passed

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

I have...

• Confirmed the correct type prefix in the PR title
• Confirmed all author checklist items have been addressed
• Confirmed that this PR does not change production code

[greptile-apps]

PR author is not in the allowed authors list.