Use either a secret or infinitime as username

Uses either the DOCKER_HUB_USERNAME secret or just infinitime as
the username for pushing to and pulling from Docker Hub.

.github/workflows/docker.yml

@@ -13,6 +13,8 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    env:
+      USERNAME: infinitime
     steps:
       - uses: actions/checkout@v3
 
@@ -20,7 +22,7 @@ jobs:
         if: github.event_name == 'push'
         uses: docker/login-action@v2
         with:
-          username: infinitime
+          username: ${{ secrets.DOCKER_HUB_USERNAME || env.USERNAME }}
           password: ${{ secrets.DOCKER_HUB_ACCESS_TOKEN }}
 
       - name: Set up QEMU
@@ -37,6 +39,6 @@ jobs:
           platforms: linux/amd64,linux/arm64
           builder: ${{ steps.buildx.outputs.name }}
           push: ${{ github.event_name == 'push' }}
-          tags: infinitime/infinitime-build:latest
-          cache-from: type=registry,ref=infinitime/infinitime-build:buildcache
-          cache-to: type=registry,ref=infinitime/infinitime-build:buildcache,mode=max
+          tags: ${{ secrets.DOCKER_HUB_USERNAME || env.USERNAME }}/infinitime-build:latest
+          cache-from: type=registry,ref=${{ secrets.DOCKER_HUB_USERNAME || env.USERNAME }}/infinitime-build:buildcache
+          cache-to: type=registry,ref=${{ secrets.DOCKER_HUB_USERNAME || env.USERNAME }}/infinitime-build:buildcache,mode=max