Synology DS2415+ docker 17.05 does not support --build-arg

Dockerfile

FROM busybox

ARG FOO

RUN env && echo $FOO

mac

agross@firiel ~/Downloads/test
$ docker --version
Docker version 17.09.0-ce, build afdb6d4

agross@firiel ~/Downloads/test
$ docker build --no-cache --build-arg FOO=42 .
Sending build context to Docker daemon  2.048kB
Step 1/3 : FROM busybox
 ---> 6ad733544a63
Step 2/3 : ARG FOO
 ---> Running in 5905f0697f4e
 ---> 95f0e024daad
Removing intermediate container 5905f0697f4e
Step 3/3 : RUN env && echo $FOO
 ---> Running in 61b56de4056b
HOSTNAME=61b56de4056b
SHLVL=1
HOME=/root
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
FOO=42
PWD=/
42
 ---> 22d87fd43a05
Removing intermediate container 61b56de4056b
Successfully built 22d87fd43a05

syno

$ docker --version
Docker version 17.05.0-ce, build 34ed091-synology

agross@NAS ~/test
$ docker build --no-cache --build-arg FOO=42 .
Sending build context to Docker daemon  2.048kB
Step 1/3 : FROM busybox
 ---> 6ad733544a63
Step 2/3 : ARG FOO
 ---> Running in 98bb102c933b
 ---> 85a15904257a
Removing intermediate container 98bb102c933b
Step 3/3 : RUN env && echo $FOO
 ---> Running in 1b1f68c48247
HOSTNAME=98bb102c933b
SHLVL=1
HOME=/root
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/

 ---> e961ab1e9c53
Removing intermediate container 1b1f68c48247
Successfully built e961ab1e9c53

is there a solution?

FROM busybox

ARG FOO
ENV FOO=$FOO

RUN env && echo $FOO

doesn't look nice but works

This is insane! Been banging my head on this for over an hour now!