Actions: remove actions/checkout from Docker build (#575)

BuildKit checks out the repository itself DawnbrandBots/bastion-bot#415

Author: kevinlul

.github/workflows/docker.yml

@@ -29,7 +29,6 @@ jobs:
       group: ${{ github.workflow }}-${{ github.ref }}
       cancel-in-progress: true
     steps:
-      - uses: actions/checkout@v4
       - uses: docker/setup-buildx-action@v3
       - uses: actions/cache@v4
         with: