sanitize Docker's config.json and report errors

[guillaume-d]

ATM Docker's config.json undergoes no plausibility checks, and there is no feedback if something there is fundamentally wrong (that would be a generalization of #5):

Some ideas for things that could be checked:

• "credsStore" : "desktop" should be thrown silently away if present: happened IRL, and it was really confusing
• valid JSON syntax