Add GitHub Action for Composer validation.

Author: Potherca

.github/workflows/quality-checks.yml

@@ -0,0 +1,15 @@
+---
+name: Quality Assurance jobs
+
+on:
+  - push
+  - pull_request
+
+jobs:
+  composer-validate:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - uses: "docker://composer"
+        with:
+          args: composer validate --strict --working-dir=solid/