Pin openresty rpm to 1.21.4-1.el8

Author: tkan145

Makefile

@@ -69,7 +69,7 @@ export COMPOSE_PROJECT_NAME
 #     docker push quay.io/3scale/apicast-ci:openresty-X.Y.Z-{release_number}
 # * update .circleci/config.yaml openresty executor with the image URL
 .PHONY: dev-build
-dev-build: export OPENRESTY_RPM_VERSION?=1.21.4
+dev-build: export OPENRESTY_RPM_VERSION?=1.21.4-1.el8
 dev-build: export LUAROCKS_VERSION?=3.12.0
 dev-build: IMAGE_NAME ?= apicast-development:latest
 dev-build: ## Build development image