Malformed Docker Registry URL fails silently

[Charlie Pitkin]

Issue summary:
Malformed by leaving out the //
DOCKER_REGISTRY_URL = 'https:335883679137.dkr.ecr.us-west-2.amazonaws.com/wearefine/base_ruby'

Code:
The error occurs on this line: https://github.com/wearefine/rails-jenkins-shared-libraries/blob/master/vars/railsDocker.groovy#L31

Proposed fix:
Check that the URL has the correct format when checking for the presence of the value