Fix adding/replacing a grammar (#4097)

* Licensed needs a full path now

* Add docker installed/running guard

* Docker is required for adding/replacing grammars

* Use more elegant method

Hat-tip to @Alhadis 🎩

script/add-grammar

@@ -84,6 +84,10 @@ unless $url
   exit 1;
 end
 
+# Exit early if docker isn't installed or running.
+log "Checking docker is installed and running"
+command('docker', 'ps')
+
 # Ensure the given URL is an HTTPS link
 parts    = parse_url $url
 https    = "https://#{parts[:host]}/#{parts[:user]}/#{parts[:repo]}"
@@ -107,6 +111,7 @@ log "Confirming license"
 if repo_old
   command('script/licensed')
 else
+  repo_new = File.absolute_path(repo_new)
   command('script/licensed', '--module', repo_new)
 end