Delete docker.yml

docker-hub上でビルドするようにした

.github/workflows/docker.yml

-name: Docker build
-on:
-  push:
-    branches:
-      - master
-      - develop
-  pull_request:
-
-jobs:
-  build:
-    runs-on: ubuntu-latest
-
-    steps:
-    - uses: actions/checkout@v2
-    - name: Pull previous build result (for cache)
-      run: docker pull misskey/misskey:latest
-    - name: Build docker container
-      run: docker build --cache-from misskey/misskey:latest -t misskey/misskey .