Lots of warnings are displayed when assembly.mode is tar or tgz when building on windows

[zoltanersek]

Addressing issue #591

[rhuss]

Thanks a lot. I'm currently on the road but will be back soon. Will check out the PR ASAP.

[rhuss]

Thanks a lot, makes sense to add this option.

Could you please do me the following favours:

• Please sign-off your commit as described in Contributing
• Please add some documentation along with the possible values for longTarFileMode. These should go into https://github.com/fabric8io/docker-maven-plugin/blob/master/src/main/asciidoc/inc/build/_assembly.adoc

Thanks !

[zoltanersek]

Thanks. I've signed the commit and added the documentation.

[rhuss]

Thanks a lot, will include the PR in the next release (probably end of the week)