ci(docker): add linux/riscv64 support for Docker image builds

Adds riscv64 to DOCKER_ARCHS in Makefile and un-ignores
.build/linux-riscv64 in .dockerignore.

Fixes #3311

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

Author: benevolentshrine

.dockerignore

@@ -6,3 +6,4 @@
 !.build/linux-arm64
 !.build/linux-ppc64le
 !.build/linux-s390x
+!.build/linux-riscv64

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