fix: quote variable substitution to prevent whitespace issue

Again the tracker variable should not have embeded whitespace but....

Courtesy of https://www.shellcheck.net/

Author: rouilj

scripts/Docker/roundup_healthcheck

@@ -4,4 +4,4 @@
 # returns last one for testing that server is up. Does not test
 # each tracker.
 tracker=$(ps -ef | sed -ne '/roundup-server/s/^.*\s\(\w*\)=.*$/\1/p')
-wget -q -O /dev/null --no-verbose http://localhost:8080/${tracker:-demo}/
+wget -q -O /dev/null --no-verbose http://localhost:8080/"${tracker:-demo}"/