Fixes#31
Credit to @tadr0 for spotting it[1]. Problem introduced when adding quotes here[2].
I tested this from my branch, and my container can now restart cleanly again:
```
2020-08-12 23:30:42,646 WARN received SIGTERM indicating exit request
2020-08-12 23:30:42,647 INFO waiting for backuppc, lighttpd, watchmails to die
2020-08-12 23:30:42,648 INFO stopped: watchmails (terminated by SIGTERM)
2020-08-12 23:30:42,650 INFO stopped: lighttpd (exit status 0)
2020-08-12 23:30:43,652 INFO stopped: backuppc (exit status 0)
2020-08-12 23:30:46,304 INFO Set uid to user 0 succeeded
2020-08-12 23:30:46,309 INFO RPC interface 'supervisor' initialized
2020-08-12 23:30:46,310 INFO supervisord started with pid 1
2020-08-12 23:30:47,314 INFO spawned: 'backuppc' with pid 13
2020-08-12 23:30:47,317 INFO spawned: 'lighttpd' with pid 14
2020-08-12 23:30:47,319 INFO spawned: 'watchmails' with pid 15
2020-08-12 23:30:48,426 INFO success: backuppc entered RUNNING state, process has stayed up for > than 1 seconds (startsecs)
```
[1] https://github.com/adferrand/docker-backuppc/issues/31#issuecomment-672891314
[2] 95034418c1 (diff-46bd948566ee03504c6421cadd8047faR119)
and add a clarification to README.
On Ubuntu 16.04.3, mounting /etc/localtime from host to container
does not work. The container exits with error in entrypoint.sh.
