Bump docker/build-push-action from 4 to 6

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 4 to 6.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Commits](docker/build-push-action@v4...v6)

---
updated-dependencies:
- dependency-name: docker/build-push-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/docker.yml

@@ -77,7 +77,7 @@ jobs:
 
       - name: Build and push TACC image for main
         if: contains(github.event_name, 'push')
-        uses: docker/build-push-action@v4
+        uses: docker/build-push-action@v6
         with:
           context: ./docker/rayleigh-tacc/
           cache-from: type=registry,ref=geodynamics/rayleigh-buildenv-bionic
@@ -90,7 +90,7 @@ jobs:
 
       - name: Build and push TACC image for release
         if: contains(github.event_name, 'release')
-        uses: docker/build-push-action@v4
+        uses: docker/build-push-action@v6
         with:
           context: ./docker/rayleigh-tacc/
           cache-from: type=registry,ref=geodynamics/rayleigh-buildenv-bionic