v0.2

CHANGES:

• Add support for nested / multi-stage builds (@talex5 #48 #49).
  This allows you to use a large build environment to create a binary and then
  copy that into a smaller runtime environment. It's also useful to get better caching
  if two things can change independently (e.g. you want to build your software and also
  a linting tool, and be able to update either without rebuilding the other).

• Add healthcheck feature (@talex5 #52).

  • Checks that Docker is running.
  • Does a test build using busybox.

• Clean up left-over runc containers on restart (@talex5 #53).
  If btrfs crashes and makes the filesystem read-only then after rebooting there will be stale runc directories.
  New jobs with the same IDs would then fail.

• Remove dependency on dockerfile (@talex5 #51).
  This also allows us more control over the formatting
  (e.g. putting a blank line between stages in multi-stage builds).

• Record log output from docker pull (@talex5 #46).
  Otherwise, it's not obvious why we've stopped at a pull step, or what is happening.

• Improve formatting of OBuilder specs (@talex5 #45).

• Use seccomp policy to avoid necessary sync operations (@talex5 #44).
  Sync operations are really slow on btrfs. They're also pointless,
  since if the computer crashes while we're doing a build then we'll just throw it away and start again anyway.
  Use a seccomp policy that causes all sync operations to "fail", with errno 0 ("success").
  On my machine, this reduces the time to apt-get install -y shared-mime-info from 18.5s to 4.7s.
  Use --fast-sync to enable to new behaviour (it requires runc 1.0.0-rc92).

• Use a mutex to avoid concurrent btrfs operations (@talex5 #43).
  Btrfs deadlocks enough as it is. Don't stress it further by trying to do two things at once.

Internal changes:

• Improve handling of file redirections (@talex5 #46).
  Instead of making the caller do all the work of closing the file descriptors safely, add an FD_move_safely mode.

• Travis tests: ensure apt cache is up-to-date (@talex5 #50).