…queue up from the default playlist instead of don't play anything

- Environment variables
  - `BOT_TOKEN` : Discord Token
  - `BOT_OWNER` : Owner ID of Discord

- Minimize the image size (alpine)

- com.typesafe version update from 1.3.2 -> 1.4.2
for supporting the HOCON env var overriding feature

- env: `CONFIG_FORCE_token` and `CONFIG_FORCE_owner`
must be set

- the program will generate a folder named JMusicBot under current working directory
that mounted to the internal docker directory.
Any config files generated by the bot will be sync with the folder

- some small enhancements have been made in BotConfig.java and OtherUtil.java
for it to work with Docker feature