Bump docker/build-push-action from 5.4.0 to 6.0.0 (#1073)

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 5.4.0 to 6.0.0.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Commits](docker/build-push-action@v5.4.0...v6.0.0)

---
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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/build-tag.yml

@@ -26,7 +26,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
       - id: vars
         run: make ci--env
-      - uses: docker/build-push-action@v5.4.0
+      - uses: docker/build-push-action@v6.0.0
         with:
           tags: |
             ghcr.io/beryju/gravity:stable

.github/workflows/build.yml

@@ -26,7 +26,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
       - id: vars
         run: make ci--env
-      - uses: docker/build-push-action@v5.4.0
+      - uses: docker/build-push-action@v6.0.0
         with:
           tags: |
             ghcr.io/beryju/gravity:latest