[nix-improvements] special case for Docker's "386" architecture

shell.nix

@@ -45,8 +45,8 @@ in
       function to_docker {
         executor  \
           --force \
-          --customPlatform="''${DOCKER_PLATFORM}" \
-          --destination "''${CONTAINER_NAME}:''${CONTAINER_TAG}" \
+          --customPlatform="$(echo "''${DOCKER_PLATFORM}" | sed 's/i386/386/')" \
+          --destination "$(echo "''${CONTAINER_NAME}" | sed 's/i386/386/'):''${CONTAINER_TAG}" \
           --context dir://`pwd` \
           --verbosity=debug
       }