Warn if variable declaration is not closed

The following Dockerfile cannot be built but it passes the validator.
```Dockerfile
FROM scratch
ARG aaa=${bbb
```
```
$ dockerfile-utils lint Dockerfile
$
```
```
$ docker build .
Sending build context to Docker daemon  1.775MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : ARG aaa=${bbb
failed to process "${bbb": missing ':' in substitution
```