ci: add x86-64 alpine to the multiarch matrix

Author: flavorjones

.github/workflows/ci.yml

@@ -278,6 +278,7 @@ jobs:
           - {platform: arm-linux,     dockerfile: alpine, from_image: arm32v6/alpine }
           - {platform: arm-linux,     dockerfile: debian, from_image: navikey/raspbian-bullseye }
           - {platform: x86-linux,     dockerfile: alpine, from_image: i386/alpine }
+          - {platform: x86_64-linux,  dockerfile: alpine, from_image: alpine }
           - {platform: x86_64-linux,  dockerfile: centos, from_image: amd64/centos }
     runs-on: ubuntu-latest
     steps: