docker machine fails to create instances on AWS when the gitlab runner token contains an underscore

Expected behavior

when I use docker machine with gitlab runner to create AWS instances I need to supply the parameter MachineName, which requires a %s in it. That will be replaced by the runner token.

Relevant logs and/or screenshots

root@ip-10-2-0-109:~# cat /etc/gitlab-runner/config.toml
concurrent = 10
check_interval = 5

[[runners]]
  name = "auto-scale-runner-aws"
  limit = 15
  url = "http://1.2.3.4/"
  token = "afz_7WP1Y2-515gXU43m"
  executor = "docker+machine"
  [runners.docker]
    tls_verify = false
    image = "docker:latest"
    privileged = true
    disable_cache = false
    volumes = ["/cache"]
    services = ["docker:dind"]
    shm_size = 0
  [runners.cache]
    Type = "s3"
    ServerAddress = "s3-eu-west-1.amazonaws.com"
    BucketName = "gitlab"
  [runners.machine]
    IdleCount = 2
    IdleTime = 2700
    MaxBuilds = 100
    MachineDriver = "amazonec2"
    MachineName = "%s-gitlab"
    MachineOptions = [
      "amazonec2-region=eu-west-1",
      "amazonec2-vpc-id=something",
      "engine-storage-driver=overlay",
      "amazonec2-ami=ami-6d48500b",
      "amazonec2-tags=environment,staging,docker-machine,true,project,gitlab-runner",
      "amazonec2-subnet-id=something",
      "amazonec2-security-group=security_group_docker_machine",
      "amazonec2-request-spot-instance",
      "amazonec2-spot-price=0.50"]
    OffPeakTimezone = ""
    OffPeakIdleCount = 0
    OffPeakIdleTime = 0

Jan 16 10:18:37 ip-10-2-0-109 gitlab-runner[16411]: time="2018-01-16T10:18:37Z" level=error msg="Error creating machine: Invalid hostname specified. Allowed hostname chars are: 0-9a-zA-Z . -" driver=amazonec2 name="runner-afz_7wp1-1516097917-2eee1465-gitlab" operation=create

Environment description

root@ip-10-2-0-109:~# docker info
Containers: 0
 Running: 0
 Paused: 0
 Stopped: 0
Images: 0
Server Version: 17.12.0-ce
Storage Driver: overlay2
 Backing Filesystem: extfs
 Supports d_type: true
 Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file logentries splunk syslog
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 89623f28b87a6004d4b785663257362d1658a729
runc version: b2567b37d7b75eb4cf325b77297b140ea686ce8f
init version: 949e6fa
Security Options:
 apparmor
 seccomp
  Profile: default
Kernel Version: 4.4.0-1041-aws
Operating System: Ubuntu 16.04.3 LTS
OSType: linux
Architecture: x86_64
CPUs: 2
Total Memory: 3.858GiB
Name: ip-10-2-0-109
ID: WVPR:R2HD:XIMP:ZX6O:XCR3:5W7V:YLIN:SJDW:BUS4:BESU:YLAB:M74A
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false

WARNING: No swap limit support

ii  gitlab-ce                        10.3.3-ce.0                                amd64        GitLab Community Edition (including NGINX, Postgres, Redis)
ii  gitlab-runner                    10.3.0                                     amd64        GitLab Runner

Used GitLab Runner version

root@ip-10-2-0-109:~# gitlab-runner --version
Version:      10.3.0
Git revision: 5cf5e19a
Git branch:   10-3-stable
GO version:   go1.8.5
Built:        Fri, 22 Dec 2017 08:47:19 +0000
OS/Arch:      linux/amd64