ci: pin GitHub Actions to SHA-256 commits

Pin all action references to specific commit SHAs to prevent supply-chain
attacks. Each pin is annotated with a human-readable version tag for clarity.
Versions stay within the existing major version to avoid breaking changes.

Author: n8mgr

.github/workflows/rebuild.yml

@@ -14,26 +14,26 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
 
       - name: Login to GHCR
-        uses: docker/login-action@v3
+        uses: docker/login-action@c94ce9fb468520275223c153574b00df6fe4bcc9 # v3.7.0
         with:
           registry: ghcr.io
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Extract metadata
         id: meta
-        uses: docker/metadata-action@v5
+        uses: docker/metadata-action@c299e40c65443455700f0fdfc63efafe5b349051 # v5.10.0
         with:
           images: ghcr.io/${{ github.repository }}
           tags: |
             type=raw,value=latest
             type=sha,format=short
 
       - name: Build and push
-        uses: docker/build-push-action@v6
+        uses: docker/build-push-action@10e90e3645eae34f1e60eeb005ba3a3d33f178e8 # v6.19.2
         with:
           context: .
           push: true