[IDEA] New ENV variable to allow different borg command to run when RUN_ON_STARTUP = true

[televisi]

As per discussion on #295.

When Environment variable RUN_ON_STARTUP = true is set.
During docker start, it will run the following command:

docker-borgmatic/entry.sh

Line 81 in 42e312f

/usr/local/bin/borgmatic --stats -v 0 2>&1

Perhaps a good idea to introduce a new docker environment that allow users to set the initial borg command with something else.

Thank you