ADD incorrectly flags JSON's closing bracket as an invalid argument

[rcjsuen]

Docker will happily build the following Dockerfile but the validator incorrectly flags the closing bracket as an error. Caused by implementing #17.

FROM alpine
ADD [ "t1.txt", "t2.txt", "./" ]

$ docker build .
Sending build context to Docker daemon  1.696MB
Step 1/2 : FROM alpine
 ---> 3fd9065eaf02
Step 2/2 : ADD [ "t1.txt", "t2.txt", "./" ]
 ---> bbf5b39e1dc5
Successfully built bbf5b39e1dc5

$ dockerfile-utils lint Dockerfile
Line: 2
ADD [ "t1.txt", "t2.txt", "./" ]
                               ^
Error: When using ADD with more than one source file, the destination must be a directory and end with a / or a \