Fix docker-gen breaking on new docker version (See #335)

[NicolasDorier]

This PR is scanning for the full container ID by getting the short container id via HOSTNAME, then trying to fetch the rest of it in "/proc/self/cgroup" or "/proc/self/mountinfo".

So no breaking change to old versions of docker.

Alternative to #335

Note I did not tested on new docker, but I tested on old one and nothing break.

[NicolasDorier]

Tested on new docker, and this fix the issue.

[NicolasDorier]

This comment indicates there is a corner case that might not be handled by this PR. #335 (comment)

[buchdag]

@NicolasDorier does the comment in #335 means that docker-gen now cannot get its own ID if you set a custom host name for the container ?

[NicolasDorier]

@buchdag I don't really know, I don't use a custom host name, so I did not tried. Just mentioning it here. The comment mention /etc/hostname but I use $HOSTNAME so that might be OK, I just did not tried.

[buchdag]

@NicolasDorier @jwilder this breaks the tests:

--- FAIL: TestGetCurrentContainerID_DockerCE (0.00s)
    context_test.go:49: id mismatch: got 18862cabc2e0d24142cf93c46ccb6e070c2ea7b996c81c0311ec, exp 18862cabc2e0d24142cf93c46ccb6e070c2ea7b996c81c0311ec0309abcbcdfb
FAIL
exit status 1
FAIL    github.com/jwilder/docker-gen   0.596s

[buchdag]

Unfortunately I confirm that this PR broke docker-gen containers that uses a custom host name via --hostname (and result in empty upstream blocks on the rendered nginx config).

[buchdag]

Steps to reproduce:

$ docker network create nginx-proxy

$ docker run --detach \
> --name nginx-proxy \
> --network nginx-proxy \
> --publish 80:80 \
> --publish 443:443 \
> --volume conf:/etc/nginx/conf.d \
> nginx

$ docker run --detach \
> --name nginx-proxy-gen \
> --network nginx-proxy \
> --hostname dockergen \
> --volumes-from nginx-proxy \
> --volume /path/to/nginx.tmpl:/etc/docker-gen/templates/nginx.tmpl:ro \
> --volume /var/run/docker.sock:/tmp/docker.sock:ro \
> jwilder/docker-gen \
> -notify-sighup nginx-proxy -watch -wait 5s:30s /etc/docker-gen/templates/nginx.tmpl /etc/nginx/conf.d/default.conf

$ docker run --detach \
> --name proxyed-app \
> --network nginx-proxy \
> --env "VIRTUAL_HOST=domain.tld" \
> nginx

$ sleep 10; docker exec nginx-proxy cat /etc/nginx/conf.d/default.conf

The resulting /etc/nginx/conf.d/default.conf:

# If we receive X-Forwarded-Proto, pass it through; otherwise, pass along the
# scheme used to connect to this server
map $http_x_forwarded_proto $proxy_x_forwarded_proto {
  default $http_x_forwarded_proto;
  ''      $scheme;
}
# If we receive X-Forwarded-Port, pass it through; otherwise, pass along the
# server port the client connected to
map $http_x_forwarded_port $proxy_x_forwarded_port {
  default $http_x_forwarded_port;
  ''      $server_port;
}
# If we receive Upgrade, set Connection to "upgrade"; otherwise, delete any
# Connection header that may have been passed to this server
map $http_upgrade $proxy_connection {
  default upgrade;
  '' close;
}
# Apply fix for very long server names
server_names_hash_bucket_size 128;
# Default dhparam
# Set appropriate X-Forwarded-Ssl header
map $scheme $proxy_x_forwarded_ssl {
  default off;
  https on;
}
gzip_types text/plain text/css application/javascript application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript;
log_format vhost '$host $remote_addr - $remote_user [$time_local] '
                 '"$request" $status $body_bytes_sent '
                 '"$http_referer" "$http_user_agent"';
access_log off;
		ssl_protocols TLSv1.2 TLSv1.3;
		ssl_ciphers 'ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384';
		ssl_prefer_server_ciphers off;
# HTTP 1.1 support
proxy_http_version 1.1;
proxy_buffering off;
proxy_set_header Host $http_host;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection $proxy_connection;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $proxy_x_forwarded_proto;
proxy_set_header X-Forwarded-Ssl $proxy_x_forwarded_ssl;
proxy_set_header X-Forwarded-Port $proxy_x_forwarded_port;
# Mitigate httpoxy attack (see README for details)
proxy_set_header Proxy "";
server {
	server_name _; # This is just an invalid value which will never trigger on a real hostname.
	listen 80;
	access_log /var/log/nginx/access.log vhost;
	return 503;
}
# domain.tld
upstream domain.tld {
}
server {
	server_name domain.tld;
	listen 80 ;
	access_log /var/log/nginx/access.log vhost;
	location / {
		proxy_pass http://domain.tld;
	}
}

[hanhpv]

Docker version 20.10.5, build 55c4c88966
Got an empty upstream: