Add riscv64 (#906)

(cherry picked from commit 39df180)

.github/workflows/ci.yml

@@ -44,8 +44,9 @@ jobs:
             linux/amd64
             linux/arm/v7
             linux/arm64
-            linux/s390x
             linux/ppc64le
+            linux/riscv64
+            linux/s390x
           )
           SAVE_IFS="$IFS"
           IFS=","

.github/workflows/release.yml

@@ -22,7 +22,7 @@ jobs:
     needs:
       - ci
     env:
-      PLATFORMS: linux/amd64,linux/arm/v7,linux/arm64,linux/s390x,linux/ppc64le
+      PLATFORMS: linux/amd64,linux/arm/v7,linux/arm64,linux/s390x,linux/riscv64,linux/ppc64le
       PACKAGE: ghcr.io/${{github.repository_owner}}/base-ubuntu
     steps:
       - name: Checkout

README.md

@@ -19,8 +19,9 @@ docker build \
 ### Arguments
 
 - `TARGETPLATFORM` - Set by [Docker Buildx] automatically. Currently supported
-  platforms are `amd64`, `arm/v7`, `arm64`, `ppc64le` and `s390x`. This should
-  be set automatically even if building locally using standard [Docker CLI].
+  platforms are `amd64`, `arm/v7`, `arm64`, `ppc64le` `riscv64`, and `s390x`.
+  This should be set automatically even if building locally using standard
+  [Docker CLI].
 - `LANGUAGE` - The language code that is set globally for the system. Defaults
   to `en_US`. See [locale] for more details.
 - `ENCODING` - Character encoding that is set globally for the system. Defaults