RF: Use usermod instead of addgroup in the Dockerfile template.

[arokem]

Hopefully closes #305.

[maouw]

I got the tests to succeed using moto[cloudformation]==4.1.5.

It started succeeding with moto==4.1.10. I chose this because 4.1.11 is where there were some changes to handling duplicate tags, according to the changelog:

• ECR: put_image(): now behaves correctly on duplicate images with duplicate tags

When I first tried with moto==4.1.10, test_DockerImage succeeded. However, test_pars_with_new_vpc started to fail at cloudknot/tests/test_pars.py:298:

An error occurred (DependencyViolation) when calling the DeleteStack operation: The vpc vpc-b2ae771a has dependencies and cannot be deleted.

I went back to the moto changelog, and looked for where there were any potential changes to CloudFormation, and found 4.1.6 changed how stack deletion was handled:

• CloudFormation now supports deletion of AWS::EC2::Subnet, AWS::EC2::VPC
• CloudFormation now supports variable mapping inside "Fn::Sub"
• CloudFormation: delete_stack() now adheres to "DeletionPolicy": "Retain" set for individual resources

So, I set moto==4.1.5 and it worked!

There is one warning that we might want to check out:

cloudknot/tests/test_knot.py::test_knot
  . . . moto/ec2/models/instances.py:140: PendingDeprecationWarning: Could not find AMI with image-id:None, in the near future this will cause an error.
  Use ec2_backend.describe_images() to find suitable image for your test

[arokem]

Superseded by #307