build.sh: Use DOCKER to allow local command overrides

On Debian, the binary is installed as docker.io [1] to avoid confusion
with the existing window manager docker package [2].

While you can override DOCKER, it would be nice if we just worked out
of the box for everyone.  Use 'command -v' [3] and the temporary
DOCKER_IO to setup the following chain:

1. If DOCKER is already set (e.g., by the user) use it.
2. Fall back to docker.io, if we find it in the PATH.
3. Fall back to docker.

The logic is that if docker.io exists, it's almost certainly what we
want.  If docker exists in the path, it's more ambiguous.

[1]: http://packages.debian.org/unstable/docker.io
[2]: http://packages.debian.org/unstable/docker
[3]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html