This commit is multi-fold as it also refactors internally context and logger
as interfaces so it is easier to plug luet as a library externally.
Introduces a garbage collector (related to #227) but doesn't handle yet
parallelism.
Closes#265
Add api call which uses go-container registry to create OCI images from
standard tar archives.
Consume new API when generating final images instead of docker building them
and adapts/add tests as necessary.
This change now allows to carry over xattrs to final images.
Fixes#266
