Provide PDO_SOURCE_ROOT default for docker to allow zero-config build

[g2flyer]

Minor change to allow docker builds out-of-the-box without any configuration/environment variable definition. This default for PDO_SOURCE_ROOT also hides an error for undefined PDO_SOURCE_ROOT which is very unintuitive to figure out if you haven't run into this issue beforehand.

[bvavala]

This assumes make is running within the docker folder, which is not the case if you run with the -f option outside of it -- this does not happen to us usually, but it's a possibility.

Suggested change

	PDO_SOURCE_ROOT ?= $(shell readlink -f ..)
	mkfile_path := $(dir $(abspath $(lastword $(MAKEFILE_LIST))))
	PDO_SOURCE_ROOT ?= $(realpath $(mkfile_path)../)

The proposed change makes the computation of the parent folder dependent on the makefile location itself.

[g2flyer]

Ooops, given that i usually use make -C docker .. i should have known better. Thanks for catching. Pushed corresponding update merging in your changes.

[cmickeyb]

by the way... that $(lastword $(MAKEFILE_LIST)) does not work if there is a space in the path name. so i'm not sure that there is a "perfect" way to do this.

[g2flyer]

by the way... that (lastword(MAKEFILE_LIST)) does not work if there is a space in the path name. so i'm not sure that there is a "perfect" way to do this.

good point. But somebody on unix/linux having spaces in paths is calling for trouble. Stackoverflow also argues that make does not support filenames with whitespace in them.. $(CURDIR) would work for the -C case but alas would not work for the -f case. I would tend to leave it as-is? Definitely better than my original version and are we sure everything else works when there are spaces in paths? never mind: $(CURDIR) just would have been a more elegant but equivalent form to my original version..

[cmickeyb]

one small request... we define DOCKER_DIR farther down in the Makefile. Can you move that up and set it instead of mkkfile_path (to be clear... it was ALREADY in the makefile, farther down, with the comment about why it was not being set with MAKEFILE_LIST). That being said... its set using MAKEFILE_LIST in the pdo-contracts repo.

[cmickeyb]

this looks great!