build, docs: Improve macOS building

- New docker command for quickly running tasks in a Linux environment.
- Updated docs with macOS specific details.
- Fixed some test issues.
- Add (fallible) macOS test target for Travis.

test/shell/file-mode.sh

@@ -1,9 +1,15 @@
 #!/bin/bash -e
 
+if [[ $(uname) == "Darwin" ]] ; then
+	# https://github.com/purpleidea/mgmt/issues/33
+	echo "This test is broken on macOS, skipping!"
+	exit
+fi
+
 set -x
 
 # run till completion
-timeout --kill-after=40s 35s ./mgmt run --yaml file-mode.yaml --converged-timeout=5 --no-watch --tmp-prefix &
+$timeout --kill-after=40s 35s ./mgmt run --yaml file-mode.yaml --converged-timeout=5 --no-watch --tmp-prefix &
 pid=$!
 wait $pid	# get exit status
 e=$?