github: Switch DOCKER_USERNAME to a variable instead of a secret

This will allow avoiding masking out this string from the log output.

Author: mstorsjo

.github/workflows/docker.yml

@@ -50,7 +50,7 @@ jobs:
         if: ${{inputs.login}}
         uses: docker/login-action@v3
         with:
-          username: ${{secrets.DOCKER_USERNAME}}
+          username: ${{vars.DOCKER_USERNAME}}
           password: ${{secrets.DOCKER_PASSWORD}}
       - name: Build Docker images
         uses: docker/build-push-action@v5
@@ -99,7 +99,7 @@ jobs:
         if: ${{inputs.login}}
         uses: docker/login-action@v3
         with:
-          username: ${{secrets.DOCKER_USERNAME}}
+          username: ${{vars.DOCKER_USERNAME}}
           password: ${{secrets.DOCKER_PASSWORD}}
       - name: Build Docker images
         uses: docker/build-push-action@v5
@@ -134,7 +134,7 @@ jobs:
       - name: Log in to Docker Hub
         uses: docker/login-action@v3
         with:
-          username: ${{secrets.DOCKER_USERNAME}}
+          username: ${{vars.DOCKER_USERNAME}}
           password: ${{secrets.DOCKER_PASSWORD}}
       - name: Create final image
         run: |