Allow running tomcat as non-root

[jeanpommier]

It is a good practice to allow running a container as unprivileged user.

To preserve backward compatibility and provide flexibility, this feature is added at the entrypoint level and is set using environment variables.

• RUN_UNPRIVILEGED=true activates the unprivileged mode with default uid:gid as 999:999
• RUN_WITH_USER_UID allows to set the uid used for tomcat user
• RUN_WITH_USER_UID allows to set the gid used for tomcat group
• CHANGE_OWNERSHIP_ON_FOLDERS accepts a space-separated list of folder on which a chmod will be run, changing (recursively) the ownership for the tomcat user.