MavenInDocker: Check if image is already built and reuse it

[schnatterer]

Less log cluttering on jenkins. Faster builds.

Note: Add a version of the image (like, 1, 2, 3, 4) so a rebuilt can be forced when the dockerfile is changed.

Example to illustrate basic concept:

private static final int DOCKER_FILE_VERSION=1

// ...
    def mvn(String args) {
// ....
if (!dockerImagePresent("ces-build-lib/maven/$dockerImageVersion-$DOCKER_FILE_VERSION")) {
 script.docker.build("ces-build-lib/maven/$dockerImageVersion-$DOCKER_FILE_VERSION", createDockerfilePath())
}

When writeDockerFile() is changed, DOCKER_FILE_VERSION is increased by one, forcing a rebuild of the archive.

[schnatterer]

As of #7 now obsolete.