Support referencing nginx-gen container indirectly through another environment variable

[ento]

Motivation

I'm running this container alongside nginx-gen in Amazon ECS. The docker-gen container gets assigned a mangled name like "ecs-tws3-24-nginx-gen-aa9cf4babdd593ae5e00", which means I can't simply set NGINX_DOCKER_GEN_CONTAINER=nginx-gen.

What I ended up doing is linking those containers and having Docker assign and expose a namespaced name as an environment variable like:

NGINX_GEN_NAME=/ecs-tws3-23-nginx-letsencrypt-a6d9b293fc808bf7fd01/nginx-gen

What this PR does

This PR adds the ability to specify the name of this environment variable instead of directly specifying docker-gen's container ID or name.

[JrCs]

@ento can you fix the code with the reviews i have made ?

[JrCs]

Can you made the changes from my reviews ?

[JrCs]

It 's better to use
if [[ -n
than
if [[ ! -z

[JrCs]

Sed is not necessary here. Instead use

export NGINX_DOCKER_GEN_CONTAINER=$(echo ${!NGINX_DOCKER_GEN_LINKED_CONTAINER_ENVVAR#/*})

[JrCs]

I think the best is to use labels to references each others containers ?

[teohhanhui]

I think this is closed by #181.

[helderco]

@teohhanhui #181 is only for the joined jwilder/nginx-proxy container, not separate jwilder/docker-gen and nginx.

But #231 is another approach using a label and should solve this as well.

[ento]

Confirmed #231 works with Amazon ECS by using the dockerLabels property:

{
  {
    "image": "nginx",
    "dockerLabels": {
      "com.github.jrcs.letsencrypt_nginx_proxy_companion.nginx_proxy": ""
    },
    ...
  },
  {
    "image": "ento/docker-nginx-gen-with-tpl",
    "dockerLabels": {
      "com.github.jrcs.letsencrypt_nginx_proxy_companion.docker_gen": ""
    },
    ...
  },
  ...
}