Provide image for Gentoo

Was looking at:

• https://github.com/saltstack-formulas/php-formula/pull/205

Which itself was based on:

• https://github.com/saltstack-formulas/php-formula/pull/156 (merged by @javierbertoli!)

Tried to get a Gentoo docker image running but it is a very difficult proposition. Unless I'm missing something, there's no way this is going to be achieved as part of the Travis matrix, the build takes forever.

However, if we can prepare a pre-salted image manually, could we just maintain and upload this separately?