Add environment vars to configure Memcached on Graphite

[lucasdaddiego]

This allows to configure the Memcached options to Graphite, using environment variables, that can be easily added to the container with the -e "ENVVAR=VALUE" option to the docker run command like this:

docker run -d\
--name graphite\
--restart=always\
-p 80:80\
-p 2003-2004:2003-2004\
-p 2023-2024:2023-2024\
-p 8125:8125/udp\
-p 8126:8126\
-e "MEMCACHE_HOST=127.0.0.1:11211"\  # Memcached host. Separate by comma more than one servers.
-e "CACHE_DURATION=60"\              # in seconds
hopsoft/graphite-statsd

Note that this only allows the config to be used by Graphite, this NOT installs a Memcached server in the container.

Also, you can specify more than one memcached server, using commas: -e "MEMCACHE_HOST=127.0.0.1:11211,10.0.0.1:11211"

[hopsoft]

This looks great. Will you add a small section to the README that describes the usage? I'll get this merged once the README is ready. Thanks.

[lucasdaddiego]

Done! Hope you like it.