Fix docker run error without attached /srv/var

[paskal]

Before:

➜  docker run umputun/remark42:dev
init container
set timezone America/Chicago (Sat Apr 11 11:19:28 CDT 2020)
custom APP_UID not defined, using default uid=1001
execute /srv/init.sh
prepare environment
/srv/init.sh failed

After:

➜ docker run umputun/remark42:dev
init container
set timezone America/Chicago (Sat Apr 11 15:30:59 CDT 2020)
custom APP_UID not defined, using default uid=1001
execute /srv/init.sh
prepare environment
ERROR: /srv/var doesn't exist, which means that state of the application
ERROR: will be lost on container stop or restart.
ERROR: Please mount local directory to /srv/var in order for it to work.
/srv/init.sh failed