Add exclusion for linux-riscv64 in .dockerignore

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

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>
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