Update OpenSSL!

[Former user]

I just set up my new mail server and I really love poste.io. Great work so far! I am planing to purchase a comercial release if everything goes well.

However I found one serious problem. Please keep the base Ubuntu image up-to-date! There were several serious security issues with openssl lately! The best would be to set automatic rebuild triggers at DockerHub. So everytime Ubuntu gets updated, the poste.io container will be rebuild with the latest security fixed and updates.

Thank You!

[Loic Lambiel]

Same concern here #155

[SH]

Updated. Thanks! I admit that build system is not ideal right now and will be changed in future...