ci: allow devbox bootstrap egress

[komer3]

Summary

• allow the Jetify and Nix bootstrap endpoints needed by devbox in CI workflows
• allow index.docker.io for the docker build workflow under harden-runner block mode
• keep e2e on harden-runner block by pre-creating an IPv4-only shared Docker kind network before ctlptl/kind runs
• allow *.linodeobjects.com:443 for object-storage e2e coverage under harden-runner block mode

Problem

The original failure was e2e-test (all) / all-e2e-tests stopping in Install devbox, which pointed to missing bootstrap egress rather than a cache issue.

After fixing the devbox bootstrap allowlist, e2e still failed before the suite started while Docker/kind created the management cluster:
failed to advertise addresses: write ip ::1->ff02::1: sendmsg: operation not permitted

Our goal here was to keep harden-runner on egress-policy: block, not relax it to audit.

Approach

• add the small set of bootstrap endpoints needed for devbox
• avoid the remaining Docker/kind startup failure by pre-creating the shared Docker kind network as IPv4-only in the e2e workflows
• keep the fix workflow-local and minimal rather than broadening the policy
• after the kind startup issue was fixed, add *.linodeobjects.com:443 for the object-storage e2e traffic that showed up in the next block-mode run

Validation

• Install devbox passed under block after the allowlist update: https://github.com/linode/cluster-api-provider-linode/actions/runs/25401633761
• quick e2e under block reached and ran the suite after the IPv4-only kind network change: https://github.com/linode/cluster-api-provider-linode/actions/runs/25455607906
• broader validation also passed with e2e-selector=all under block: https://github.com/linode/cluster-api-provider-linode/actions/runs/25457168014

[komer3]

Left on a comment on what seems like a similar type of multi cast issue on the harden runner repo. We can track any fixes for our use case and remove the work around in future.

Harden Runner issue: step-security/harden-runner#228 (comment)