build configuration

[errordeveloper]

As more features are being added and requested, support for config files becomes essential.

Requirements

• only one config per image
• must rebuild on config file changes
• copy of config file should be stored in a label
• copy of dockerfile used should also be stored in a label
• git hash of config file should be part of the image tag
• if tag is present, rebuilder should inspect config stored in the label and validate it's git hash
• once the feature is stable, it should be the only supported mode
• config file should be outside base dir
• multiple base dir can be used with the same config file as part of variants
• a custom dockerfile can be specified per variant or for all variants
• dockerfile can be specified in-line as well as by-path