added some comments

Dockerfile

+# +--------------------------------------+
+# | You need to get calibre-web first!   |
+# | Just do a 'git clone ...' and put    |
+# | Calibre-Web into './calibre-web', so |
+# | the build-command can copy it into   |
+# | the Docker-image.                    |
+# +--------------------------------------+
+
 FROM ubuntu:18.04
 
 RUN apt-get update