"x509: certificate signed by unknown authority" error when running job

Whenever my job runs using gitlab-ci-multi-runner, I get this:

Starting build...
Using Docker executor with image ruby:2.2.2 ...
Pulling docker image ruby:2.2.2 ...

Build failed with Post https://192.168.59.103:2376/images/create?fromImage=ruby%3A2.2.2: x509: certificate signed by unknown authority

I'm running on Mac OS X, using boot2docker. I can successfully run docker pull ruby:2.2.2 and docker run -it ruby:2.2.2 on my machine.

The output of boot2docker shellinit is

Writing /Users/todd/.boot2docker/certs/boot2docker-vm/ca.pem
Writing /Users/todd/.boot2docker/certs/boot2docker-vm/cert.pem
Writing /Users/todd/.boot2docker/certs/boot2docker-vm/key.pem
    export DOCKER_TLS_VERIFY=1
    export DOCKER_HOST=tcp://192.168.59.103:2376
    export DOCKER_CERT_PATH=/Users/todd/.boot2docker/certs/boot2docker-vm

My config.toml looks like

concurrent = 1

[[runners]]
  name = "lorentz.local"
  url = <URL>
  token = <TOKEN>
  limit = 0
  executor = "docker"
  builds_dir = ""
  clean_environment = false
  environment = ["DOCKER_HOST=tcp://192.168.59.103:2376", "DOCKER_CERT_PATH=/Users/todd/.boot2docker/certs/boot2docker-vm", "DOCKER_TLS_VERIFY=1"]
  shell = ""
  disable_verbose = false
  [runners.docker]
    host = "tcp://192.168.59.103:2376"
    hostname = ""
    image = "ruby:2.2.2"
    privileged = false
    disable_cache = false
    disable_pull = false
    volumes = ["/cache"]
    cache_dir = ""
    registry = ""

Is there something else I need to configure in the environment or settings for the runner?

Thank you.