Use relative paths as far as possible.

Switch from absolute paths `hostpath` to relative paths `contextpath`.
Unfortunately, for the url input javascript is necessary to resolve the
relative url.
Also see Issue #205.

examples/nginx-contextpath/README.md

@@ -69,8 +69,8 @@ services:
     image: plantuml/plantuml-server:jetty
     container_name: plantuml-server
     environment:
-      - TZ="Europe/Berlin"
-      - BASE_URL="plantuml"
+      - TZ=Europe/Berlin
+      - BASE_URL=plantuml
 
   nginx:
     image: nginx:alpine
@@ -78,7 +78,7 @@ services:
     ports:
       - "80:80"
     environment:
-      - TZ="Europe/Berlin"
+      - TZ=Europe/Berlin
     volumes:
       - ./nginx.conf:/etc/nginx/nginx.conf:ro
 ```