Remove default volume from Dockerfile

Currently, as of https://github.com/moby/moby/issues/3465
`VOLUME` points cannot be removed in inherited images.
Therefore, anybody aiming to modify the image or the
LE_CONFIG_HOME, will still have an unnamed volume created
even if it is not used. If users want real persistence,
then the volume should be configured and named at creation
time. Otherwise an new unnamed volume will be created each
time, not providing much of an advantage.

Dockerfile

@@ -69,7 +69,5 @@ else \n \
  exec -- \"\$@\"\n \
 fi" >/entry.sh && chmod +x /entry.sh
 
-VOLUME /acme.sh
-
 ENTRYPOINT ["/entry.sh"]
 CMD ["--help"]