Reports invalid destination directories in ADD and COPY commands.

According to the Dockerfile specification, if multiple sources are specified, then the destination must be a directory, and it must end with a slash '/'. Otherwise, Docker build will fail.

Examples:

  # all the commands below will fail
  ADD textA.txt textB.txt relativeDir
  ADD ["binaryA.jar", "binary2.jar", "destination"]
  COPY text3.txt text4.txt /absolute/path

After the quick-fix is applied:

  ADD textA.txt textB.txt relativeDir/
  ADD ["binaryA.jar", "binary2.jar", "destination/"]
  COPY text3.txt text4.txt /absolute/path/