travis: skip login+push for pull requests

travis/docker-build-and-push.sh

@@ -7,5 +7,11 @@ set -e
 echo .git > .dockerignore
 
 docker build --pull --no-cache --rm -t=${BASENAME} -f travis-build.Dockerfile .
+# For pull requests, travis does not add secure environment variables to the
+# environment (because pull requests could then steal their values), so skip
+# the login+push step when the variable isn’t set.
+if [ -n "${DOCKER_PASS}" ]
+then
 	docker login -e ${DOCKER_EMAIL} -u ${DOCKER_USER} -p ${DOCKER_PASS}
 	docker push ${BASENAME}
+fi