Better support built-in image overrides - part 1

[tobybellwood]

This is the first step towards #458

This PR:

• loosens the test routines checking for exact image names
• Reorders the DockerPull regex to identify the image reference given
• Support a case insensitive header check (newer haproxy/http2 headers are all lowercase now)

The actual logic to support the image overrides will be done elsewhere (and likely not by me :stuck_out_tongue:)