Docker compose HOSTNAME env works differently than in docker run

Docker Engine Compose
1

The hostname directive does not work properly in docker compose as the environment variable HOSTNAME does not contain the full domain name.

$ docker run --hostname host.dom.example.com busybox hostname
host.dom.example.com

And

$ docker run --hostname host.dom.example.com busybox printenv HOSTNAME
host.dom.example.com

But in docker compose:

$ cat > test.yml <<'EOF'
version: '2'
services:
  host.dom.example.com:
    image: busybox
    hostname: host.dom.example.com
    command: printenv HOSTNAME
EOF
$ docker-compose -f test.yml up
host.dom.example.com_1  | host

Not:

host.dom.example.com_1  | host.dom.example.com

Yet:

cat > test2.yml <<'EOF'
version: '2'
services:
  host.dom.example.com:
    image: busybox
    hostname: host.dom.example.com
    command: hostname
EOF
$ docker-compose -f test2.yml up
host.dom.example.com_1  | host.dom.example.com
2

I’ve also posted as bug to github here