Bug 1972 fixed

Now the Dockerfile.local uses Ubuntu 20.04;
The paths in the format `/[...]/python<version>/[...]` where replaced
with the new Python version (3.8 instead of 3.6)

Change-Id: Iccccecb2bd4f4d8156357dba582b0460987b060f
Signed-off-by: Pedro Escaleira <escaleira@av.it.pt>