[signer] Update action versions to silence GitHub warnings

Signed-off-by: Michael Brown <mbrown@fensystems.co.uk>

Author: mcb30

.github/workflows/signer.yml

@@ -26,25 +26,25 @@ jobs:
         uses: actions/checkout@v6
 
       - name: Login
-        uses: docker/login-action@v3
+        uses: docker/login-action@v4
         with:
           registry: ghcr.io
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Set up builder
-        uses: docker/setup-buildx-action@v3
+        uses: docker/setup-buildx-action@v4
 
       - name: Build
-        uses: docker/build-push-action@v6
+        uses: docker/build-push-action@v7
         with:
           context: signer
           push: true
           tags: |
             ${{ env.reponame }}/${{ env.pkgname }}:draft-${{ github.sha }}
 
       - name: Build (pcscd)
-        uses: docker/build-push-action@v6
+        uses: docker/build-push-action@v7
         with:
           context: signer
           target: pcscd
@@ -53,7 +53,7 @@ jobs:
             ${{ env.reponame }}/${{ env.pcscdname }}:draft-${{ github.sha }}
 
       - name: Build (checker)
-        uses: docker/build-push-action@v6
+        uses: docker/build-push-action@v7
         with:
           context: signer
           target: check-signer