README: Package 'libbtrfs-dev' doesn't exist on Ubuntu < 18.10

[fhemberger]

I'm building skopeo in a container on Ubuntu 18.04 LTS, the package libbtrfs-dev only exists in versions 10.10 and above.

(Also at least registry to registry copying seems to work fine without it.)

[rhatdan]

Can't you eliminate btrfs from the build?

[fhemberger]

I just left out installing the dependency and so far, it seems to work fine.
Not sure why you list it as build dependency and in which case it's needed.

[rhatdan]

I think it is needed if you were building with containers/storage == btrfs.

[mtrmac]

IIRC the dependency has always been there, it’s just that before 18.10 it was not necessary to install libbtrfs-dev for it (#664). I haven’t looked very hard for other ways to get this working on 18.10, to be fair.

PRs are welcome if there is some other dependency that can be used to pull in the required headers, as long as it works on 18.10 and we can keep the README and Dockerfile.build consistent; it’s been hard enough to keep Dockerfile.build up-to-date when most development of this package is Fedora-based, so keeping a completely separate and likely-to-be-outdated instructions in README for other versions of Ubuntu is not very attractive.