Warn if ADD has more than two arguments and the last one is not a directory

[rcjsuen]

FROM alpine
ADD x y z

$ docker build .
Sending build context to Docker daemon  1.695MB
Step 1/2 : FROM alpine
 ---> 3fd9065eaf02
Step 2/2 : ADD x y z
When using ADD with more than one source file, the destination must be a directory and end with a /

Same thing as #14 but for the ADD instruction.