Adjust Docker build platforms in Github actions (#165)

This commit updates the listed platforms for Docker builds in two Github
action workflow files: 'main.yml' and 'publish.yml'. It specifies now to
run the builds on both 'linux/amd64' and 'linux/arm64' platforms.

.github/workflows/publish.yml

@@ -76,7 +76,7 @@ jobs:
           tags: |
             jbzoo/csv-blueprint:latest
             jbzoo/csv-blueprint:${{ github.event.release.tag_name }}
-          platforms: linux/amd64,linux/arm64/v8,linux/386
+          platforms: linux/amd64,linux/arm64
           build-args: |
             VERSION=${{ github.event.release.tag_name }}