Allow overriding filter flags in clear-docker-cache script

Description

Our gitlab runners run buildx jobs to produce container images. This results with lots of volumes with labels other than default coded in the clear-docker-cache script.

Proposal

We would like to re-use the script providing alternative filter label with environment variable. Example:

FILTER_FLAG='label=buildx.volume.cache=true' /usr/share/gitlab-runner/clear-docker-cache

Links to related issues and merge requests / references

I didn't find anything related.