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/push_complement_image.yml

@@ -33,29 +33,29 @@ jobs:
       packages: write
     steps:
       - name: Checkout specific branch (debug build)
-        uses: actions/checkout@v4
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         if: github.event_name == 'workflow_dispatch'
         with:
           ref: ${{ inputs.branch }}
       - name: Checkout clean copy of develop (scheduled build)
-        uses: actions/checkout@v4
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         if: github.event_name == 'schedule'
         with:
           ref: develop
       - name: Checkout clean copy of master (on-push)
-        uses: actions/checkout@v4
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         if: github.event_name == 'push'
         with:
           ref: master
       - name: Login to registry
-        uses: docker/login-action@v3
+        uses: docker/login-action@74a5d142397b4f367a81961eba4e8cd7edddf772 # v3.4.0
         with:
           registry: ghcr.io
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
       - name: Work out labels for complement image
         id: meta
-        uses: docker/metadata-action@v5
+        uses: docker/metadata-action@902fa8ec7d6ecbf8d84d538b9b233a880e428804 # v5.7.0
         with:
           images: ghcr.io/${{ github.repository }}/complement-synapse
           tags: |