Upgrades app Dockerfile for Ubuntu 18.04

[joestrong]

A solution to issue #813 , and supersedes pull request #815

Changes:

• Upgrades docker ubuntu version from 16.04 to 18.04 (16.04 no longer maintained)
• Passes DEBIAN_FRONTEND=noninteractive flag during install due to a timezone selection prompt
• Removes mkdir /run/php, as this seems to be created now by default
• Removes password from RSA key, as a password less than 4 characters in length is no longer supported