Use "jekyll/jekyll:3.8.6" so it is locked, rather than "latest", refs #8370

Julian van den Berkmortel · Julian van den Berkmortel · commit a456333b24d0 · 2020-12-15T00:10:19.000+01:00
diff --git a/docker-compose.yaml b/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
