Silence shellcheck warnings (#279)

* SC1091: the `config` file might not be present, which is normal.
* SC2086: Double quote to prevent globbing and word splitting.

Tested clean output using: `find -name "*.sh" -exec shellcheck -x {} \;`

build-docker.sh

@@ -15,6 +15,7 @@ fi
 set -e
 
 if [ -f config ]; then
+	# shellcheck disable=SC1091
 	source config
 fi