Run Scala3doc CI on self-hosted node

Author: romanowski

.github/workflows/scala3doc.yaml

@@ -7,8 +7,10 @@ on:
   pull_request:
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, Linux]
+    container: lampepfl/dotty:2020-04-24
     timeout-minutes: 20
+    
     steps:
 
       - uses: actions/checkout@v2