Chore: Dependency pinning & Depdendabot introduction (#436)

1. Pins the Github Action workflow hashes under `.github/workflows` to
full-length commit SHAs.
2. Adds a base level `.github/dependabot.yml` config to keep these (and
other go/Docker) deps up to date.

## Motivation and Context
- Github's [security
guide](https://docs.github.com/en/actions/reference/security/secure-use#using-third-party-actions)
recommends using the immutable SHA of an action when consuming third
party actions.
- Depdendabot makes ongoing dep house keeping manageable.

.github/workflows/release.yml

@@ -14,23 +14,23 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 #v4
         with:
           fetch-depth: 0
 
       - name: Set up Go
-        uses: actions/setup-go@v5
+        uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5 #v5
         with:
           go-version: '1.24.x'
 
       - name: Install cosign
-        uses: sigstore/cosign-installer@v3
+        uses: sigstore/cosign-installer@d58896d6a1865668819e1d91763c7751a165e159 #v3
 
       - name: Install Syft
         uses: anchore/sbom-action/download-syft@v0.20.5
 
       - name: Run GoReleaser
-        uses: goreleaser/goreleaser-action@v6
+        uses: goreleaser/goreleaser-action@e435ccd777264be153ace6237001ef4d979d3a7a #v6
         with:
           distribution: goreleaser
           version: v2.12.0
@@ -43,13 +43,13 @@ jobs:
     needs: goreleaser
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955 #v4
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
+        uses: docker/setup-buildx-action@e468171a9de216ec08956ac3ada2f0791b6bd435 #v3
 
       - name: Log in to Container Registry
-        uses: docker/login-action@v3
+        uses: docker/login-action@184bdaa0721073962dff0199f1fb9940f07167d1 #v3
         with:
           registry: ghcr.io
           username: ${{ github.actor }}
@@ -57,7 +57,7 @@ jobs:
 
       - name: Extract metadata
         id: meta
-        uses: docker/metadata-action@v5
+        uses: docker/metadata-action@c1e51972afc2121e065aed6d45c65596fe445f3f #v5
         with:
           images: ghcr.io/${{ github.repository }}
           tags: |
@@ -65,7 +65,7 @@ jobs:
             type=raw,value=latest
 
       - name: Build and push Docker image
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@ca052bb54ab0790a636c9b5f226502c73d547a25 #v5
         with:
           context: .
           file: ./Dockerfile