TIKA-4703: Fix Docker Hub secret name DOCKERHUB_USERNAME -> DOCKERHUB_USER (#2781)

The Apache Tika Docker Hub org secret is named DOCKERHUB_USER, not
DOCKERHUB_USERNAME. Fix all three occurrences across docker-snapshot.yml
and docker-release.yml so the login step can authenticate successfully.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: nddipiazza

.github/workflows/docker-release.yml

@@ -45,7 +45,7 @@ jobs:
       - name: Login to Docker Hub
         uses: docker/login-action@4907a6ddec9925e35a0a9e82d7399ccc52663121 # v4.1.0
         with:
-          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          username: ${{ secrets.DOCKERHUB_USER }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
       - name: Build and push tika-server minimal
@@ -104,7 +104,7 @@ jobs:
       - name: Login to Docker Hub
         uses: docker/login-action@4907a6ddec9925e35a0a9e82d7399ccc52663121 # v4.1.0
         with:
-          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          username: ${{ secrets.DOCKERHUB_USER }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
       - name: Prepare tika-grpc Docker build context

.github/workflows/docker-snapshot.yml

@@ -57,7 +57,7 @@ jobs:
       - name: Login to Docker Hub
         uses: docker/login-action@4907a6ddec9925e35a0a9e82d7399ccc52663121 # v4.1.0
         with:
-          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          username: ${{ secrets.DOCKERHUB_USER }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
       # --- tika-server (minimal) ---