[patch] docker: Explicitly install python-futures

[patch] docker: Explicitly install python-futures

For x86_64, the Saltstack bootstrapping scripts fetch a custom build
of `python-tornado` DEB package, which being arch-specific won't be
picked up on AArch64 (and a similar version from Ubuntu Xenial repos
will be used instead).

Although the Ubuntu package works just fine, it lacks a hard
dependency on `python-futures`, which became mandatory in Salt
2017.7.8, see [1].

Explicitly install `python-futures` inside cfg01 Docker container
during build, which will be a no-op on x86_64 and fix the issue on
AArch64.

[1] https://github.com/saltstack/salt/issues/50220

Change-Id: Ie4aad064572788c0852aaf398f21437b456becbe
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>