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

[rcjsuen]

FROM alpine
COPY Dockerfile Dockerfile2 /root

$ docker build .
Sending build context to Docker daemon  1.878MB
Step 1/2 : FROM alpine
 ---> 3fd9065eaf02
Step 2/2 : COPY Dockerfile Dockerfile2 /root
When using COPY with more than one source file, the destination must be a directory and end with a /

If a COPY has more than two arguments, the last argument must be a directory. The Docker builder will check for this by validating that the last argument ends with the OS's path segment separator (either / or \).