build: use unique hash for alpine-base bootstrap to avoid cache collisions

[rucoder]

Description

Add a workflow_dispatch workflow that builds and pushes eve-alpine-base for all three platforms (amd64, arm64, riscv64), then automatically rebuilds eve-alpine with the new hash.

The workflow uses a unique hash (content-hash-git-short-rev) to avoid cache collisions when the Dockerfile content hasn't changed but the image needs rebuilding (e.g. after Alpine point releases).

The eve-alpine build step uses --disable-cache to prevent linuxkit from reusing a potentially stale local cache that may reference the old alpine-base image with fewer platforms.

Pipeline:

1. Build alpine-base for amd64/arm64/riscv64 (with --force + unique hash)
2. Create multi-arch manifest for alpine-base
3. Commit updated hash to pkg/alpine/Dockerfile
4. Build eve-alpine for amd64/arm64/riscv64 (with --force --disable-cache)
5. Create multi-arch manifest for eve-alpine

How to test and validate this PR

1. Merge this PR
2. Trigger the workflow from Actions tab
3. Verify both images have all 3 platforms:

   docker buildx imagetools inspect lfedge/eve-alpine-base:<new-hash>
   docker buildx imagetools inspect lfedge/eve-alpine:<new-hash>
   

4. Verify the auto-commit updated pkg/alpine/Dockerfile

Tested on https://github.com/rene/eve fork with mikemzed/ images.

Changelog notes

No user-facing changes.

PR Backports

• 16.0-stable: No, build infrastructure change
• 14.5-stable: No, build infrastructure change
• 13.4-stable: No, build infrastructure change

Checklist

• I've provided a proper description

• I've written the test verification instructions

• I've set the proper labels to this PR

• I've checked the boxes above, or I've provided a good reason why I didn't
  check them.