Support additional apk feeds

[andibraeu]

this PR contains the following changes:

• allow to use apk repositories
• adds repositories to the default repositories instead of replacing them
• make network mode configurable, as pasta is sometime not available

[a-gave]

+1 for making the existing api 'repositories' able to both add extra repositories and replace existing ones, thanks from me!

[andibraeu]

I could add a flag or enum to the API to enable "replace" or "append" original repos

[andibraeu]

extended the api...