GUACAMOLE-146: Allow webapp context to be specified in Docker with environment variable.

guacamole-docker/bin/start.sh

@@ -329,7 +329,7 @@ END
 start_guacamole() {
 
     # Install webapp
-    ln -sf /opt/guacamole/guacamole.war /usr/local/tomcat/webapps/
+    ln -sf /opt/guacamole/guacamole.war /usr/local/tomcat/webapps/${WEBAPP_CONTEXT:-guacamole}.war
 
     # Start tomcat
     cd /usr/local/tomcat