image file ownership and mode is too naive

[cyphar]

We create files in the image using standard os.Create and similar calls, resulting in the behaviour that new files created in an image layout directory are owned by the user running umoci with a mode determined by the umask that umoci happened to run with.

However, since umoci repack and umoci unpack almost always require sudo to operate, it seems that changing the ownership to match the ownership of something like index.json or the layout directory itself would make umoci a bit easier to use.

I noticed this when replying to #528.

[tych0]

sudo exports SUDO_{G,U}ID, which is what I used to fix this type of thing in stacker.