NPE in docker:save when supplying file name without directory

[wrosenuance]

Description

If the docker.save.file system property is used to override the output location for docker:save and the property value is just a file name without a directory, the plugin throws an NPE.

Info

• d-m-p version : 0.29-SNAPSHOT
• Maven version (mvn -v) : 3.5.4

The issue is in ensureSaveDir(), which does not check for a null result from getParentFile(), which happens when the File does not specify a directory.

    private void ensureSaveDir(String fileName) throws MojoExecutionException {
        File saveDir = new File(fileName).getParentFile();
        if (!saveDir.exists()) {
            if (!saveDir.mkdirs()) {
                throw new MojoExecutionException("Can not create directory " + saveDir + " for storing save file");
            }
        }
    }

I will submit a PR to fix, it's easy.