chore: clean up github actions workflows

.github/workflows/dotnet-docker.yml

@@ -0,0 +1,47 @@
+name: Build and push Docker image
+on:
+  push:
+    paths:
+    - '.github/workflows/docker.yml'
+    - 'ci/dotnet-version.sh'
+    - 'Myriad/**'
+    - 'PluralKit.API/**'
+    - 'PluralKit.Bot/**'
+    - 'PluralKit.Core/**'
+
+jobs:
+  build:
+    name: '.net docker build'
+    runs-on: ubuntu-latest
+    permissions:
+      packages: write
+    if: github.repository == 'PluralKit/PluralKit'
+    steps:
+      - uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.CR_PAT }}
+      - uses: actions/checkout@v2
+      - run: echo "BRANCH_NAME=${GITHUB_REF#refs/heads/}" | sed 's|/|-|g' >> $GITHUB_ENV
+
+      - name: Extract Docker metadata
+        id: meta
+        uses: docker/metadata-action@v3
+        with:
+          images: ghcr.io/${{ github.repository }}
+          tags: |
+            type=raw,value=${{ env.BRANCH_NAME }}
+            type=raw,value=${{ github.sha }}
+            type=raw,value=latest,enable=${{ github.ref == 'refs/heads/main' }}
+          flavor: |
+            latest=false
+
+      - uses: docker/build-push-action@v2
+        with:
+          # https://github.com/docker/build-push-action/issues/378
+          context: .
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          cache-from: type=registry,ref=ghcr.io/pluralkit/pluralkit:${{ env.BRANCH_NAME }}
+          cache-to: type=inline