GITLAB_TRUSTED_PROXIES is evaluated inside a yaml list item instead of expanded as an array

In the default case, NGINX is used inside a container and has a single IP address. But multiple default proxies could be used, or, for instance, by running nginx or apache directly on the containing host. We do the latter. GITLAB_TRUSTED_PROXIES must be set to the containing host's IP address, but this can change.
Proposal:

1. Change the yaml expansion to:

  trusted_proxies: [ $GITLAB_TRUSTED_PROXIES ] 

where GITLAB_TRUSTED_PROXIES is an array of quoted strings. This will also obviate the code in functions:

  else
    exec_as_git sed -i "/{{GITLAB_TRUSTED_PROXIES}}/d" ${GITLAB_CONFIG}

since

  trusted_proxies: [ ]

is a valid expression.

2. Use a dummy name to refer to containing host's docker-generated IP address. GITLAB_TRUSTED_PROXIES will set itself to this via the default route of the container. Example:

 gitlab:
   environment:
    GITLAB_TRUSTED_PROXIES = dockerhost

If the container reaches the containing host by 172.18.0.5, then gitlab.yml will contain:

   trusted_proxies: [ '172.18.0.5' ]

To determine this in the functions script, just use this code (test in container):

route -n |awk '$1=="0.0.0.0" { print $2 }'

Pseudo-code:

gitlab_configure_trusted_proxies() {
  if [[ "${GITLAB_TRUSTED_PROXIES}" = "_dockerhost" ]]; then
    GITLAB_TRUSTED_PROXIES=$( route -n |awk '$1=="0.0.0.0" { print $2 }' )
 fi
 echo "Configuring gitlab::trusted_proxies... for ${GITLAB_TRUSTED_PROXIES}"
 update_template ${GITLAB_CONFIG} GITLAB_TRUSTED_PROXIES
}

Start by reading the functions script and the GitLab configuration template containing GITLAB_TRUSTED_PROXIES. Reproduce the current YAML expansion in a container, then determine how empty, multiple, and host-derived proxy values should be represented. Done means trusted proxies render as a valid array and the configuration path handles the proposed host address behavior.