ci: debug the apt proxy on ubuntu 26.04

[viceice]

Changes

Temporary debug workflow, don't merge: on #7487 (runners on ubuntu-26.04) the container builds can't reach the apt proxy (host.docker.internal:8000, connection refused). The workflow runs on 24.04 and 26.04 and shows the listening ports, the squid-deb-proxy service, the firewall rules and whether the proxy is reachable from the host and from a container.

Context

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• This closes an existing Issue, Closes: #
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