Try again in serial if tests fail in parallel

Many of the upstream tests are really resource intensive, so if they fail while running in parallel on a given host, chances are decent they'll pass in serial, so we should fall back once before we give up.

Author: tianon

1/alpine/Dockerfile

@@ -106,7 +106,8 @@ RUN set -eux; \
 # see https://github.com/docker-library/memcached/pull/54#issuecomment-562797748 and https://bugs.debian.org/927461 for why we have to munge openssl.cnf
 	sed -i.bak 's/SECLEVEL=2/SECLEVEL=1/g' /etc/ssl/openssl.cnf; \
 {{ ) else "" end -}}
-	make test PARALLEL="$nproc"; \
+# try the tests in parallel first, but many of them are resource-intensive, so fall back to serial
+	make test PARALLEL="$nproc" || make test; \
 {{ if env.variant == "debian" then ( -}}
 	mv /etc/ssl/openssl.cnf.bak /etc/ssl/openssl.cnf; \
 {{ ) else "" end -}}