Update Makefile

Adds riscv64 to DOCKER_ARCHS to enable building and publishing 
linux/riscv64 container images.

Fixes #3311

Previously attempted in #3312 but closed when the author deleted 
their repository. That PR had received LGTM from a member.

Signed-off-by: Benevolent  <proffersor45@gmail.com>

Author: benevolentshrine

Makefile

@@ -15,7 +15,7 @@
 all::
 
 # Needs to be defined before including Makefile.common to auto-generate targets
-DOCKER_ARCHS ?= amd64 armv7 arm64 ppc64le s390x
+DOCKER_ARCHS ?= amd64 armv7 arm64 ppc64le s390x riscv64
 
 include Makefile.common