Build release images with DOCKER_CLI_EXPERIMENTAL enabled

In order to use buildx with docker versions prior to v20.10 experimental
features must be enabled. Setting at build time ensures that they are
in case they have not already been at the environment scope.

Signed-off-by: hasheddan <georgedanielmangum@gmail.com>

build/lib/release.sh

@@ -383,7 +383,7 @@ EOF
           echo "COPY nsswitch.conf /etc/" >> "${docker_file_path}"
         fi
 
-        "${DOCKER[@]}" buildx build \
+        DOCKER_CLI_EXPERIMENTAL=enabled "${DOCKER[@]}" buildx build \
           --platform linux/"${arch}" \
           --load ${docker_build_opts:+"${docker_build_opts}"} \
           -q \