ability to set file mode (when copying from Windows host to *nix container)

[glebsts]

MountableFile.getUnixFileMode() asks for 'unix:mode', and in case of non-posix system falls back to default 0100644, and sets that as file mode inside tar archive. If I want file to be writeable inside container (i.e. by logger in my jetty app), I need to do some additional trick with chmod.
I would like instead to set file mode with copy() command, i.e.
builder.copy("entryname", "path_in_container", 777)

[glebsts]

#467 please review

[glebsts]

actually found it to be useful in unix as well - I don't need to care which permissions file has in my host filesystem, in order to have specific permissions in container.