Fix unbound `DOCKER_TRACE` var in `bin/docker-test-proxy`
I forgot to export this since the script no longer sources `. bin/_docker.sh`.
set -eu
+# When set, causes docker's build output to be emitted to stderr.
+export DOCKER_TRACE="${DOCKER_TRACE:-}"
+
if [ $# -ne 0 ]; then
echo "no arguments allowed for $(basename $0), given: $@" >&2
exit 64
