Can not append values to PATH environment variable

I have gitlab-ci-multi-runner 1.1.0 (a23a25ab) with shell executor on Ubuntu Server 14.04, and I am trying to add some environment variables with JDK and Apache Ant.

Guided by this doc I added following line to /etc/gitlab-runner/config.toml:

environment = ["JAVA_HOME=/opt/jdk8", "ANT_HOME=/opt/ant"]

And tried with following script:

script: echo $ANT_HOME

It prints the value of ANT_HOME variable.

Then I changed environment to this:

environment = ["JAVA_HOME=/opt/jdk8, "ANT_HOME=/opt/ant", "PATH=/opt/jdk8/bin:/opt/ant/bin:$PATH"]

And I got this error:

gitlab-ci-multi-runner 1.1.0 (a23a25a)
Using Shell executor...
Running on ubuntu-trusty-x86-runner...
/bin/bash: line 22: mkdir: command not found

ERROR: Build failed: exit status 1

It looks like PATH gets totally overwritten. Am I doing something wrong?