Support SSL certs

Defining SSL_CERT and SSL_KEY in a containers env will enable SSL for
that container.

Re #48.
Use mozilla recommended TLS setup

See https://wiki.mozilla.org/Security/Server_Side_TLS#Nginx.

Re #48.
Optionally enable client cert authentication

To enable, set CLIENT_CA_CRT to a name in /ssl containing the signing
certificate.

Re #48.