Fix #87: Add PHP7.2 (#84)

* Add PHP7.2 preview

* Use rc- images

* Updated 7.2.0RC6 to released

* Replace libpng12-dev with libpng-dev

* Remove preview notice

Merge pull request #79 from slashrsm/increase_post_size

Increase max upload size.

Merge pull request #76 from drupal-docker/feat/soap-ext

Add php SOAP extension

Merge pull request #74 from drupal-docker/feat/bcmath

Add bcmath php extension

Merge pull request #63 from drupal-docker/deprecate/54-and-55

Deprecate php 5.4 and 5.5