Properly proxy/trap signals like SIGTERM

[scara]

If I look at the process list after starting a container from this image, sqlservr is not listed as PID 1:

# docker exec -it 5714b39eb637 ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0   4500   652 ?        Ss   09:55   0:00 /bin/sh -c /docker-entrypoint.sh
root         7  0.0  0.0  18020  1388 ?        S    09:55   0:00 bash /docker-entrypoint.sh
root         9  0.2  0.4  80220 17564 ?        Sl   09:55   0:00 /opt/mssql/bin/sqlservr
root        14  3.8 17.4 3001844 676044 ?      Sl   09:55   0:05 /opt/mssql/bin/sqlservr
root       180  0.0  0.0  34416  1452 pts/0    Rs+  09:57   0:00 ps aux

Signals won't be properly trapped and stopping this container will be slower, w/o giving mssql the ability to properly shutdown (not really an issue for dev/test).

Besides, having used the shell form for CMD will add an extra shell launch (no shell variable expansion here so quite useless).

I'll propose a PR.