Enhance Disk Usage Reporting

[elfkuzco]

We cannot accurately report disk usage because the Python Docker SDK does not expose writable layer size or image size. Pending the approval of docker/docker-py#3370, a temporary workaround in #1491 is to only consider the size of mounted volumes which covers most large writes (e.g ZIM files, Zimcheck results, etc.). However, this may miss meaningful usage in the writable layer and it also does not account for image size.

[benoit74]

One example where the disk usage is significantly wrong due to this issue is sotoki scraper where many temporary files are written in the writable layer and not in the mounted output volume (which is normal).

[elfkuzco]

Should we revert back to the initial decision of hooking into the lower level of the Docker SDK to get what we need?

[benoit74]

Could you develop what hooking into the lower level of the Docker SDK means?

[elfkuzco]

See #1553 for an implementation that uses the lower level docker API to get the stats.