CI: use docker buildx build

Fixes the warning:

```
DEPRECATED: The legacy builder is deprecated and will be removed in a future release.
            Install the buildx component to build images with BuildKit:
            https://docs.docker.com/go/buildx/
```

.github/workflows/main.yml

@@ -43,7 +43,7 @@ jobs:
 
       - name: Build Docker Image
         run: |
-          docker build . \
+          docker buildx build . \
             --cache-from artifactory.galois.com:5025/pate/pate:${GITHUB_REF//\//\-} \
             --cache-from artifactory.galois.com:5025/pate/pate:refs-heads-master \
             -t pate