Compose: Unsupported config option: 'extends'

[horsemanV]

I'm having a hard time getting the extends option to work properly in docker-compose in a situation where it worked previously:

File structure:
Docker
--conf
----docker
------docker-compose.yaml
------common.yaml
------environment
--------brand.env
------sites
--------brand.docker-compose.yaml
------.env
----nginx
--data
----initdb.d
----mysql
--sites
----brand

In the docker directory running docker-compose -f docker-compose.yaml -f common.yaml -f ./sites/brand.docker-compose.yaml config
yields the following:
ERROR: The Compose file '././sites/brand.docker-compose.yaml' is invalid because: Unsupported config option for services.brand.nginx: 'extends' Unsupported config option for services.brand.wordpress: 'extends'

Here are the files:

docker version
Client
Version: 1.13.1
API version: 1.26
Go version: go1.7.5
Git commit: 092cba3
Built: Wed Feb 8 08:47:51 2017
OS/Arch: darwin/amd64

Server:
Version: 1.13.1
API version: 1.26 (minimum version 1.12)
Go version: go1.7.5
Git commit: 092cba3
Built: Wed Feb 8 08:47:51 2017
OS/Arch: linux/amd64
Experimental: true

docker info
Containers: 2
Running: 0
Paused: 0
Stopped: 2
Images: 10
Server Version: 1.13.1
Storage Driver: aufs
Root Dir: /var/lib/docker/aufs
Backing Filesystem: extfs
Dirs: 85
Dirperm1 Supported: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
Volume: local
Network: bridge host ipvlan macvlan null overlay
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: aa8187dbd3b7ad67d8e5e3a15115d3eef43a7ed1
runc version: 9df8b306d01f59d3a8029be411de015b7304dd8f
init version: 949e6fa
Security Options:
seccomp
Profile: default
Kernel Version: 4.9.8-moby
Operating System: Alpine Linux v3.5
OSType: linux
Architecture: x86_64
CPUs: 2
Total Memory: 1.952 GiB
Name: moby
ID: MQRV:TZXW:YBRA:TQHV:JWDC:7MOR:D23V:KWZF:L2WA:6OPG:D6H5:2P6M
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): true
File Descriptors: 16
Goroutines: 26
System Time: 2017-02-11T00:26:39.392634547Z
EventsListeners: 1
No Proxy: *.local, 169.254/16
Registry: https://index.docker.io/v1/
Experimental: true
Insecure Registries:
127.0.0.0/8
Live Restore Enabled: false

[shin-]

extends is unfortunately no longer supported in format v3+

I just realized the documentation is extremely misleading on that point and created an issue on the docs repo to remedy it: https://github.com/docker/docker.github.io/issues/1607

Finally, as this happens to be a duplicate of #4315, I will close this issue and encourage you to pursue the discussion there as things develop.