ci: Add latest tag for ease of use

`dev` image for x86_64 and aarch64 could be pulled through command
`docker pull rustvmm/dev:latest` and for riscv64 through `docker pull
rustvmm/dev:latest-riscv` for developers/users to use/test.

Signed-off-by: Ruoqing He <[email protected]>

Author: RuoqingHe

.github/workflows/docker-publish.yml

@@ -68,7 +68,7 @@ jobs:
           # This is needed so that a manifest is created, and we can have the same
           # docker container on both x86_64 and arm64.
           platforms: linux/amd64,linux/arm64
-          tags: ${{ env.VERSION }}
+          tags: ${{ env.VERSION }},${{ env.IMAGE_NAME }}:latest
           cache-from: type=gha
           cache-to: type=gha,mode=max
 
@@ -125,7 +125,7 @@ jobs:
           file: Dockerfile.riscv64
           push: ${{ github.event_name != 'pull_request' }}
           platforms: linux/amd64
-          tags: ${{ env.VERSION }}-riscv
+          tags: ${{ env.VERSION }}-riscv,${{ env.IMAGE_NAME }}:latest-riscv
           cache-from: type=gha
           cache-to: type=gha,mode=max