Fix docker_clone_volume.sh to preserve hard-links

When using docker_clone_volume.sh,  I found that when copying a volume with influxdb data,
the clones image was bigger than the original.

cause: alpine base image contains a cp version (busybox) which  does _not_ preserve
hardlinks, even with the -a option.

This is fixed by using an ubuntu base image instead of alpine.

downside: ubuntu image is larger than alpine.

Source volume:

 influxdb_data               380.2 MB

Before:

 influxdb_data.bak           443.5 MB

After:

 influxdb_data.ubuntu.bak    380.2 MB

docker_clone_volume.sh

@@ -47,4 +47,4 @@ docker run --rm \
            -t \
            -v $1:/from \
            -v $2:/to \
-           alpine ash -c "cd /from ; cp -av . /to"
+           ubuntu bash -c "cd /from ; cp -av . /to"