Build DIB images from StackHPC package mirrors

[m-bull]

Build on #59 to make building DIB images from Ark package mirrors possible.

• Make network-manager cloud-init config less sensitive to whitespace.
• Allow a list of repo mirror URLs to be passed into the rocky-container image, alongside cleanup config.
• Add an element to disable apt mirror TLS verification, this is useful when a HTTP authenticating proxy is used in front of apt mirrors with basic auth.