feat: add image builders

[kruskall]

first step to decouple apk from the build context

Note: this isn't a breaking change (api-wise) for the build context and using a different ImageBuilder is not supported.

[panekj]

What other builds you have planned? This project is specifically made with apk in mind and if I recall correctly from what Ariadne said, non-apk project is WIP

[kruskall]

This project is specifically made with apk in mind and if I recall correctly from what Ariadne said, non-apk project is WIP

Note that this doesn't mean "non-apk" builders are supported (altought it makes it easier to add support for it), this PR is for decoupling "apk" (the command) from the build context. You could, for example, have a different image builder that uses a go implementation of apk (libapk-go ?).

EDIT: another use case would be an image builder that uses an alpine VM/container to build the image

[jdolitsky]

closing as old/inactive. please re-open if necessary