ci: make GHA cache export non-fatal (ignore-error=true)

The frontend/backend image builds + pushes succeed, then the final
'exporting to GitHub Actions Cache' step intermittently fails with
'error writing layer blob: not_found' (a known flaky type=gha cache backend
issue), failing the whole job. Add ignore-error=true to every cache-to so a
cache-write hiccup can't break an otherwise-successful, already-pushed build.

.github/workflows/docker-build.yml

@@ -134,7 +134,10 @@ jobs:
         platforms: ${{ matrix.platform }}
         labels: ${{ steps.meta-backend.outputs.labels }}
         cache-from: type=gha,scope=backend-${{ env.PLATFORM_PAIR }}
-        cache-to: type=gha,mode=max,scope=backend-${{ env.PLATFORM_PAIR }}
+        # ignore-error: a flaky GitHub Actions cache write ("error writing
+        # layer blob: not_found") must not fail an otherwise-successful build
+        # that already pushed the image.
+        cache-to: type=gha,mode=max,scope=backend-${{ env.PLATFORM_PAIR }},ignore-error=true
         outputs: ${{ steps.push-decision.outputs.push == 'true' && format('type=image,name={0}/{1},push-by-digest=true,name-canonical=true,push=true', env.REGISTRY, env.BACKEND_IMAGE_NAME) || 'type=cacheonly' }}
         build-args: |
           CACHEBUST=${{ github.run_number }}
@@ -360,7 +363,10 @@ jobs:
         platforms: ${{ matrix.platform }}
         labels: ${{ steps.meta-frontend.outputs.labels }}
         cache-from: type=gha,scope=frontend-${{ env.PLATFORM_PAIR }}
-        cache-to: type=gha,mode=max,scope=frontend-${{ env.PLATFORM_PAIR }}
+        # ignore-error: a flaky GitHub Actions cache write ("error writing
+        # layer blob: not_found") must not fail an otherwise-successful build
+        # that already pushed the image.
+        cache-to: type=gha,mode=max,scope=frontend-${{ env.PLATFORM_PAIR }},ignore-error=true
         outputs: ${{ steps.push-decision.outputs.push == 'true' && format('type=image,name={0}/{1},push-by-digest=true,name-canonical=true,push=true', env.REGISTRY, env.FRONTEND_IMAGE_NAME) || 'type=cacheonly' }}
         build-args: |
           CACHEBUST=${{ github.run_number }}

.github/workflows/install-smoke.yml

@@ -61,7 +61,8 @@ jobs:
           load: true
           tags: picpeak-backend:smoke
           cache-from: type=gha,scope=install-smoke
-          cache-to: type=gha,mode=max,scope=install-smoke
+          # ignore-error: a flaky GHA cache write must not fail the build.
+          cache-to: type=gha,mode=max,scope=install-smoke,ignore-error=true
 
       - name: Create Docker network
         run: docker network create picpeak-smoke