Rbenv command not found

When a job tries to execute it fails with the following:

gitlab-ci-multi-runner 0.4.2 (1e86428)
Using Shell executor...
stdin: is not a tty
Running on ci-runner...
Fetching changes...
Checking out f8858e83 as master...

whoami
root
echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/sbin:/sbin:/bin
rbenv local 1.9.3-p551
builds/95c60cbb/0/xxx/xxxx.sh: line 27: rbenv: command not found

Build failed with exit status 127

If I check $PATH as root on the ci-runner machine I have: /root/.rbenv/plugins/ruby-build/bin:/root/.rbenv/shims:/root/.rbenv/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

The multi-runner script is being run as a service and is started using sudo gitlab-ci-multi-runner start

The executor is 'shell', and rbenv was setup using instructions at https://gorails.com/setup/ubuntu/14.04

Any thoughts on where I'm going wrong?

Update Added /root/.rbenv/bin to $PATH but now just get

gitlab-ci-multi-runner 0.4.2 (1e86428)
Using Shell executor...
stdin: is not a tty
Running on ci-runner...
Fetching changes...
Checking out f8858e83 as master...

export PATH="/root/.rbenv/bin:$PATH"
rbenv local 1.9.3-p551
rbenv: version `1.9.3-p551' not installed

Build failed with exit status 1

Ruby 1.9.3-p551 is installed though.