build: Add ability to override images with YAML

[dave-tucker]

This commit adds a schema and parsing logic for an override.yml.
This file is applied to an already parsed Moby config and will replace
all instances of the source pattern with the substitute pattern.
This allows for easy replacement of images in a config file for
development or test.

Fixes: #95

Signed-off-by: Dave Tucker dave@dtucker.co.uk

[justincormack]

See my comments in the issue, filenames must not be special.

[dave-tucker]

Rebased and updated to add an overrides section to the YAML.

[justincormack]

you need to remove this

[dave-tucker]

good catch!

[dave-tucker]

Removed the unused const overrideFile

[dave-tucker]

Updated again to fix a bug. The AppendConfig wasn't appending the newly added Overrides section.

[dnephin]

This feature seems a bit strange to me. Why does it just blanket override every image in different sections (that happens to match)? How would you override non-image properties?

Why wouldn't overrides just be one file applies over another?

[justincormack]

@dnephin the use case is for CI, see #95. so we can build a set of packages to test in a local registry. We don't have any use case for non-image overrides, you would just change the file. Maybe there is a better way to do this...

[dnephin]

Ok, I understand the use case is pretty limited right now, but I think we should expect that more use cases will present themselves as this tool becomes more popular. We should try to design something that can be extended. I think the current design is really limited.

What do you think about supporting multiple files and applying the config of one other the other? That way every field can be updated. It would also let you override just one image if they have the same value.

[justincormack]

We already support multiple files. But they don't currently override, they are additive. In some cases we could support override, by saying that if the name was the same for onboot and services that will override.

[dave-tucker]

Closing as the design of this feature needs more thought