docker: ignore distro versioning of debootstrap

docker: ignore distro versioning of debootstrap

We do a minimum version check for the debootstrap but if the distro
has added their own minor version tick it would fail and fall-back to
the SCM version. This is sub-optimal as the latest/greatest version
may be broken at any one particular time. We fix that with a little
sed magic on the version string before passing to our ugly shell
versioning check.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>