Remove illegal Characters from Hostname

Runners may occasionally have hostname-illegal Characters – in particular the Underscore _.

I am not a Golang-Programmer, so I submit a Proposal here only:

In executors/docker/executor_docker.go alter to the following:

641 func (e *executor) createContainer(containerType string, imageDefinition common.Image, cmd []string, allowedInternalImages []string) (*types.ContainerJSON, error) {
    […]
651     hostname := e.Config.Docker.Hostname
652     if hostname == "" {
653         import('regexp')
654         re := regexp.MustCompile('[^-a-z0-9]')
655         hostname = re.ReplaceAllString(strings.ToLower(s.Build.ProjectUniqueName()), '')
656     }