make the workflow look more like the example

at https://github.com/docker/build-push-action#git-context

Author: myk002

.github/workflows/container-build.yml

@@ -1,8 +1,10 @@
 name: Build and Push Docker Image
+
 on:
   push:
     branches:
       - master
+
 jobs:
   build-and-push:
     strategy:
@@ -13,23 +15,20 @@ jobs:
           # - latest
     runs-on: ubuntu-latest
     steps:
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v3
     - name: Login to GHCR
       uses: docker/login-action@v3
       with:
         registry: ghcr.io
-        username: DFHack-Urist
+        username: ${{ github.actor }}
         password: ${{ secrets.GITHUB_TOKEN }}
-    # From https://github.com/docker/build-push-action#git-context
-    # Setting up Docker Buildx with docker-container driver is required
-    # at the moment to be able to use a subdirectory with Git context
-    - name: Set up Docker Buildx
-      uses: docker/setup-buildx-action@v3
     - name: Build and push
       uses: docker/build-push-action@v6
       with:
         context: "{{defaultContext}}:${{ matrix.IMAGE_TAG }}"
         push: true
-        tags: ghcr.io/${{ env.GHCR_NAMESPACE }}/${{ env.IMAGE_NAME }}:${{ matrix.IMAGE_TAG }}
+        tags: ${{ env.GHCR_NAMESPACE }}/${{ env.IMAGE_NAME }}:${{ matrix.IMAGE_TAG }}
 env:
   GHCR_NAMESPACE: dfhack
   IMAGE_NAME: build-env