Docker tests should build the page objects

The docker tests are currently not building the page object: we always assume that a previous build built them and so they are available from snapshot. However that might not be the case if a commit is pushed while the build is already on the queue: in that case, we might obtain some compilation failure just because we don't use the right version of the PO.
It's a pity since in that case we are using some CI time for nothing. So we should always build the PO before executing our docker tests.