Setting `GIT_DEPTH` breaks `gitlab-runner exec`

Summary

Setting GIT_DEPTH breaks gitlab-runner exec

Steps to reproduce

git clone git@gitlab.com:underyx/gitlab-runner-3217-repro.git`
gitlab-runner exec docker test`

Actual behavior

Fails with

FATAL: invalid value for variable "GIT_DEPTH"

Expected behavior

No errors please 🐱

Environment description

Trying to test a CI job locally on my dev machine.

gitlab-runner --version

Version:      10.6.0
Git revision: a3543a27
Git branch:   10-6-stable
GO version:   go1.9.4
Built:        2018-03-22T08:34:06+00:00
OS/Arch:      darwin/amd64

docker info

[...]
Server Version: 18.03.0-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: cfd04396dc68220d1cecbe686a6cc3aa5ce3667c
runc version: 4fc53a81fb7c994640722ac585fa9ca548971871
init version: 949e6fa
Security Options:
 seccomp
  Profile: default
Kernel Version: 4.9.87-linuxkit-aufs
Operating System: Docker for Mac
OSType: linux
Architecture: x86_64
[...]