Pin our GitHub Actions dependencies (#18255)

After the [recent supply chain attack](https://www.wiz.io/blog/new-github-action-supply-chain-attack-reviewdog-action-setup)
in `tj-actions/changed-files` and actions based on it, it's become clear
that relying on git tags to pin our dependencies is not enough (as tags
can simply be replaced). Therefore we need to switch to hashes.

Dependabot should continue to update these dependencies for us.

Best reviewed commit-by-commit. Though if CI passes, we're *probably*
fine.

.github/workflows/docker.yml

@@ -18,22 +18,22 @@ jobs:
     steps:
       - name: Set up QEMU
         id: qemu
-        uses: docker/setup-qemu-action@v3
+        uses: docker/setup-qemu-action@29109295f81e9208d7d86ff1c6c12d2833863392 # v3.6.0
         with:
           platforms: arm64
 
       - name: Set up Docker Buildx
         id: buildx
-        uses: docker/setup-buildx-action@v3
+        uses: docker/setup-buildx-action@b5ca514318bd6ebac0fb2aedd5d36ec1b5c232a2 # v3.10.0
 
       - name: Inspect builder
         run: docker buildx inspect
 
       - name: Install Cosign
-        uses: sigstore/cosign-installer@v3.8.1
+        uses: sigstore/cosign-installer@d7d6bc7722e3daa8354c50bcb52f4837da5e9b6a # v3.8.1
 
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
 
       - name: Extract version from pyproject.toml
         # Note: explicitly requesting bash will mean bash is invoked with `-eo pipefail`, see
@@ -43,13 +43,13 @@ jobs:
           echo "SYNAPSE_VERSION=$(grep "^version" pyproject.toml | sed -E 's/version\s*=\s*["]([^"]*)["]/\1/')" >> $GITHUB_ENV
 
       - name: Log in to DockerHub
-        uses: docker/login-action@v3
+        uses: docker/login-action@74a5d142397b4f367a81961eba4e8cd7edddf772 # v3.4.0
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
       - name: Log in to GHCR
-        uses: docker/login-action@v3
+        uses: docker/login-action@74a5d142397b4f367a81961eba4e8cd7edddf772 # v3.4.0
         with:
           registry: ghcr.io
           username: ${{ github.repository_owner }}
@@ -57,7 +57,7 @@ jobs:
 
       - name: Calculate docker image tag
         id: set-tag
-        uses: docker/metadata-action@master
+        uses: docker/metadata-action@902fa8ec7d6ecbf8d84d538b9b233a880e428804 # v5.7.0
         with:
           images: |
             docker.io/matrixdotorg/synapse
@@ -72,7 +72,7 @@ jobs:
 
       - name: Build and push all platforms
         id: build-and-push
-        uses: docker/build-push-action@v6
+        uses: docker/build-push-action@471d1dc4e07e5cdedd4c2171150001c434f0b7a4 # v6.15.0
         with:
           push: true
           labels: |