When running "wait-for-mysql.sh", the "echo 'show tables' | manage.py
dbshell" command is always considered a success, even if the database is
unreachable ($? == 0). So we replace this command by a global system
check.
This closes #3.
Full dependency install is replaced by just nodejs dependency install,
which should considerably accelerate the build step.
This closes #2.
Kudos to @sampaccoud for suggesting this!