Update Dockerfile for PF base image

The Policy Framework base image run under the "onap" user rather than
under the "root" user. This means that root access has to be enabled in
child Dockerfiles for configuraiton that requires root access.

Root access is turned off again later in the Dockerfile so that the
image will run under another user.

Issue-ID: POLICY-4558
Change-Id: I7245db2d485e72d130008155999242e15e60d611
Signed-off-by: liamfallon <liam.fallon@est.tech>