Use docker/login-action for ACR instead of azure/login

Simpler auth: use service principal clientId/password directly
with docker/login-action. Gated on ACR_LOGIN_SERVER variable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: johnmathews

.github/workflows/docker.yml

@@ -18,8 +18,6 @@ jobs:
             dockerfile: src/gateway/Dockerfile
           - image: worker
             dockerfile: src/worker/Dockerfile
-    env:
-      HAS_AZURE: ${{ secrets.AZURE_CREDENTIALS != '' }}
     steps:
       - uses: actions/checkout@v4
 
@@ -29,15 +27,13 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Log in to Azure
-        if: env.HAS_AZURE == 'true'
-        uses: azure/login@v2
-        with:
-          creds: ${{ secrets.AZURE_CREDENTIALS }}
-
       - name: Log in to ACR
-        if: env.HAS_AZURE == 'true'
-        run: az acr login -n acrdocumentstream
+        if: ${{ vars.ACR_LOGIN_SERVER != '' }}
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ vars.ACR_LOGIN_SERVER }}
+          username: ${{ secrets.ACR_CLIENT_ID }}
+          password: ${{ secrets.ACR_CLIENT_SECRET }}
 
       - uses: docker/build-push-action@v6
         with:
@@ -49,11 +45,11 @@ jobs:
             ghcr.io/johnmathews/documentstream-${{ matrix.image }}:${{ github.sha }}
 
       - name: Push to ACR
-        if: env.HAS_AZURE == 'true'
+        if: ${{ vars.ACR_LOGIN_SERVER != '' }}
         run: |
           docker tag ghcr.io/johnmathews/documentstream-${{ matrix.image }}:${{ github.sha }} \
-            acrdocumentstream.azurecr.io/${{ matrix.image }}:${{ github.sha }}
+            ${{ vars.ACR_LOGIN_SERVER }}/${{ matrix.image }}:${{ github.sha }}
           docker tag ghcr.io/johnmathews/documentstream-${{ matrix.image }}:${{ github.sha }} \
-            acrdocumentstream.azurecr.io/${{ matrix.image }}:latest
-          docker push acrdocumentstream.azurecr.io/${{ matrix.image }}:${{ github.sha }}
-          docker push acrdocumentstream.azurecr.io/${{ matrix.image }}:latest
+            ${{ vars.ACR_LOGIN_SERVER }}/${{ matrix.image }}:latest
+          docker push ${{ vars.ACR_LOGIN_SERVER }}/${{ matrix.image }}:${{ github.sha }}
+          docker push ${{ vars.ACR_LOGIN_SERVER }}/${{ matrix.image }}:latest