multi: fix Postgres on Travis

Fixes the docker build that was caused by
https://github.com/docker-library/postgres/issues/884.
Using the alpine and version 13 image avoids the problem introduced
with postgres 14 and debian bullseye.

make/testing_flags.mk

@@ -7,6 +7,7 @@ EXEC_SUFFIX =
 COVER_PKG = $$(go list -deps -tags="$(DEV_TAGS)" ./... | grep '$(PKG)' | grep -v lnrpc)
 NUM_ITEST_TRANCHES = 4
 ITEST_PARALLELISM = $(NUM_ITEST_TRANCHES)
+POSTGRES_START_DELAY = 5
 
 # If rpc option is set also add all extra RPC tags to DEV_TAGS
 ifneq ($(with-rpc),)