Improve bash quoting, and other similar best practices

This patch updates the shell scripts with some of the common best
practices, which should make them more resilient to unusual failures and
unexpected environments (in particular, directories with spaces).

Most of these were identified by shellcheck.

test/util/docker_entrypoint.sh

@@ -8,7 +8,7 @@
 # This is used for more hermetic Docker test environments.
 
 set -e
-. $(dirname ${0})/../util/lib.sh
+. "$(dirname "$0")/../util/lib.sh"
 
 init
 
@@ -45,7 +45,7 @@ export GOPROXY=off
 # Launch arguments, which come from docker CMD, as "chasquid" user.
 # Running tests as root makes some integration tests more difficult, as for
 # example Exim has hard-coded protections against running as root.
-sudo -u chasquid -g chasquid \
+sudo -u "chasquid" -g "chasquid" \
 	--set-home \
-	--preserve-env PATH=${PATH} \
+	--preserve-env PATH="$PATH" \
 	-- "$@"