Merge pull request #8386 from Serializator/lock-jekyll-docker-image-8370

Use "jekyll/jekyll:3.8.6" so it is locked, rather than "latest"

Author: hguthrie

docker-compose.yaml

@@ -2,7 +2,7 @@ version: '3.6'
 
 services:
   jekyll:
-    image: jekyll/jekyll:latest
+    image: jekyll/jekyll:3.8.6
     command: jekyll serve --watch --incremental --open-url --livereload
     ports:
       - 4000:4000