Provide Alpine and Busybox docker images

[cescoffier]

Related to #47

• Provide 'regular' and 'alpine' images
• Add integration tests
• Replace the Spotify Maven plugin by the Fabric8's one

DO NOT MERGE before working on 3.4.x.

It also require a change in the release process, as it simplifies it (no more edition of the pom.xml file required)

[cescoffier]

Also contain the fix for #50.