We now build multi-arch by manifest so we need to tell docker for which
platform we need the image.

ci: switch to using rules

See merge request !63

this will not push to docker hub when project is not alpine based
and is not master.

aports@39bd5002

Multiple fixes and features

See merge request !60

assign default values early in dabuild

Packages dir is a docker volume and the mount point will be
automatically created. Permissions will be correct from entrypoint.

- set to true to enable named volume
- set to absolute path to enable bind mount

On Alpine (and also on other dists) its not safe to share the same
abuild.conf for multiple Alpine releases and architectures. Use the
users .abuild/abuild.conf instead.

- apk supports caching of packages for reuse on next run. This is the
  prefered way and should be relativly fast compared to other package
  magenagers. This also removed the need for other named volumes which
  need to be setup and maintained on each run.
- correcly set perm of volume mount points

dabuild: doh! actually use the indicated `$DABUILD_DOCKER` command

See merge request !58

Signed-off-by: Richard Mortier <mort@cantab.net>

add dabuild.conf support

See merge request !57