Allow setting external dockerfile for policy-pap

If building with an external dockerfile, e.g. using
 mvn clean install -Pdocker -DdockerFile=/path/to/dockerfile
docker will look for project files in the same directory as the
new dockerfile. A new property is added for setting the context
path, to allow using existing project files in src/main/docker.

Issue-ID: POLICY-4334
Signed-off-by: danielhanrahan <daniel.hanrahan@est.tech>
Change-Id: Ica6984b0460e5814df56ab6679003e98b983d397