Fix docker_clone_volume.sh to preserve hard-links

[harmv]

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 to download than alpine. (~ 80MB vs 7 MB)

Source volume:

influxdb_data 380.2 MB

Before (busybox cp):

influxdb_data.bak 443.5 MB

After (ubuntu cp):

influxdb_data.ubuntu.bak 380.2 MB

NB: this pull request is not about size, but rather about correctness. A clone should be identical. Not preserving hard links, might actually break apps that depend on it. (like the influxdb database mentioned above)