Make docker image repositories actually working

Several changes are included:
- Expose ensureDir in helpers, and call it in the Docker client. In
  other implementations that was handled by CopyFile behind the scenes,
  but that's not the case here
- Create accessor in Artifact to create Artifact objects from files.
  This is handy when we have to carry over downloaded package content
  into caches when artifacts are already verified
- Fix various issues around the imagePush flag, so now trees are pushed
  forcefully each time
- Take into consideration the real artifact name when pushing single
  files in the docker image. This behavior should be changed eventually,
  because single files which aren't repository packages now are in its
  own docker image, but we should have just one that brings the required
  metadata alltogether.

pkg/compiler/interface.go

@@ -111,6 +111,7 @@ type Artifact interface {
 
 	SetFiles(f []string)
 	GetFiles() []string
+	GetFileName() string
 
 	GetChecksums() Checksums
 	SetChecksums(c Checksums)