[CI] improve on github workflows (#1889)

* [CI] making a proper github CI workflow

.github/workflows/docker.yml

@@ -0,0 +1,20 @@
+name: docker
+
+on:
+  workflow_call:
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+      - name: Build and push
+        uses: docker/build-push-action@v4
+        with:
+            context: .
+            push: false
+            tags: lnbitsdocker/lnbits-legend:latest
+            cache-from: type=registry,ref=lnbitsdocker/lnbits-legend:latest
+            cache-to: type=inline