Add support for importing local mcworld files

[itzg]

• Specified by user by an environment variable such as IMPORT_MCWORLD
• Would be a container path to the file
• Docs could suggest mounting a host directory to a container path such as /worlds
• Contents of mcworld would be unzipped. If the resulting level directory already exists, skip the rest with a logged warning.
• Identifies level name
• Sets level name environment variable to be provided to properties file management