ci: bump docker/build-push-action from v1 to v2

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from v1 to v2.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Changelog](https://github.com/docker/build-push-action/blob/e01a38bb45010cb980b1693690ccbac4df63d5e3/CHANGELOG.md)
- [Commits](docker/build-push-action@v1...e01a38b)

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/docker.yml

@@ -54,7 +54,7 @@ jobs:
           branch=${branch//\//-}
           echo "##[set-output name=tag;]$(echo ${branch})"
       - name: "Rebuild oasisprotocol/oasis-core-dev:${{ steps.determine_tag.outputs.tag }}"
-        uses: docker/build-push-action@v1
+        uses: docker/build-push-action@v2
         with:
           username: ${{ secrets.DOCKER_USERNAME }}
           password: ${{ secrets.DOCKER_ACCESS_TOKEN }}
@@ -64,7 +64,7 @@ jobs:
           path: docker/development
           always_pull: true
       - name: "Rebuild oasisprotocol/oasis-core-ci:${{ steps.determine_tag.outputs.tag }}"
-        uses: docker/build-push-action@v1
+        uses: docker/build-push-action@v2
         with:
           username: ${{ secrets.DOCKER_USERNAME }}
           password: ${{ secrets.DOCKER_ACCESS_TOKEN }}