add mtu to docker comose files in python test suite

[volatilemolotov]

Adds lower MTU (1460) to python docker-compose files due to GKE having lower mtu than default.

Partly fixes #29214

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[volatilemolotov]

R: @damccorm assigning directly as Jenkins is down

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[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[damccorm]

Thanks!