Capital letters in the name of your github repo cause mu to fail

[mattbucci]

This seems like an AWS issue so I'm not sure if mu can solve it, but maybe there is some string formatting ahead of repository creation that can be done to resolve it.

mu-repo-SquallyWebsite: EcsRepo (AWS::ECR::Repository) CREATE_FAILED Invalid parameter at 'repositoryName' failed to satisfy constraint: 'must satisfy regular expression '(?:[a-z0-9]+(?:[.-][a-z0-9]+)/)[a-z0-9]+(?:[.-][a-z0-9]+)*'' (Service: AmazonECR; Status Code: 400; Error Code: InvalidParameterException; Request ID: 6908aac2-afbf-11e8-a938-11b4b9eba67a)

https://regexr.com/3usn1

[mneil]

My guess is that your Git Repo or folder name doesn't match the naming conventions of ECR. We could rely only on the Git repo name and sanitize it.

Is closely related to #318 but is not the same issue. Linking to it since they would be good to fix together.