Long filenames not supported by Docker::Image.build_from_dir

[haines]

RubyGems' TarWriter doesn't support long filenames, which means that we can't use Docker::Image.build_from_dir to build an image that contains files with paths longer than 256 characters or filenames longer than 100 characters.

However, the docker build CLI works for these filenames.

[tlunter]

@haines unfortunately that's true. I haven't found a solution that works in ruby that supports the PaxHeaders format. If you find a solution, that'd be wonderful, but for now we're stuck with this. I don't think it's appropriate to shell out, etc, to get the tar file in this scenario.