update Dockerfile

[Crazy-Hopper]

Please remove the VOLUME spec from the Dockerfile and put the default config.toml into the /app directory.
Rationale:

• when running with docker and not specifying the -v {LOCAL_CONF_FILE}:/app/config.toml it results in an empty docker volume creation and final /app/config.toml becomes a directory;
• when running with kubernetes and trying to mount a key from configMap onto /app/config.toml results in the following error:

container_linux.go:247: starting container process caused "process_linux.go:359: container init caused "rootfs_linux.go:54: mounting \"/var/lib/docker/volumes/9ec49b6f63112a71127f92439e91e877315a2ba8691b23402a0aad0ab280cd5a/_data\" to rootfs \"/var/lib/docker/overlay2/03d74e1f0dfdfcd19b2d79a3db19c3fdf9948730304e9e03c5a0486617dc4cea/merged\" at \"/var/lib/docker/overlay2/03d74e1f0dfdfcd19b2d79a3db19c3fdf9948730304e9e03c5a0486617dc4cea/merged/app/..2018_10_28_00_54_20.203880441/config.toml\" caused \"not a directory\"""

which makes the one to mount the configMap somewhere else and specify a different config path to the running command.

[Crazy-Hopper]

Actually, in #62 I made much better than suggested above. :-)