jibri: add check for /dev/shm size

[saghul]

If the container was staryed without a /dev/shm of at least 2GB (defaults to 6MB in Docker) Chrome will behave erratically or crash.

Catch this with a tiny binary and make the container fail to start.

[saghul]

Test:

shm too small:

docker run -it --rm jitsi/jibri
[s6-init] making user provided files available at /var/run/s6/etc...exited 0.
[s6-init] ensuring user provided files have correct perms...exited 0.
[fix-attrs.d] applying ownership & permissions fixes...
[fix-attrs.d] 10-jibri: applying...
[fix-attrs.d] 10-jibri: exited 0.
[fix-attrs.d] done.
[cont-init.d] executing container initialization scripts...
[cont-init.d] 01-set-timezone: executing...
[cont-init.d] 01-set-timezone: exited 0.
[cont-init.d] 10-config: executing...
/dev/shm must be at least 2GB in size
[cont-init.d] 10-config: exited 1.
[cont-finish.d] executing container finish scripts...
[cont-finish.d] done.
[s6-finish] waiting for services.
[s6-finish] sending all processes the TERM signal.
[s6-finish] sending all processes the KILL signal and exiting.

shm ok:

docker run -it --rm --shm-size 2g jitsi/jibri
[s6-init] making user provided files available at /var/run/s6/etc...exited 0.
[s6-init] ensuring user provided files have correct perms...exited 0.
[fix-attrs.d] applying ownership & permissions fixes...
[fix-attrs.d] 10-jibri: applying...
[fix-attrs.d] 10-jibri: exited 0.
[fix-attrs.d] done.
[cont-init.d] executing container initialization scripts...
[cont-init.d] 01-set-timezone: executing...
[cont-init.d] 01-set-timezone: exited 0.
[cont-init.d] 10-config: executing...
FATAL ERROR: Jibri recorder password and auth password must be set
[cont-init.d] 10-config: exited 1.
[cont-finish.d] executing container finish scripts...
[cont-finish.d] done.
[s6-finish] waiting for services.
[s6-finish] sending all processes the TERM signal.
[s6-finish] sending all processes the KILL signal and exiting.

Ref: #1653