Dockerfile: Avoid pip 21.3 version preventing working container override

Without this the container override would not install properly thus failing to start.

Related to #3685 (closed) Related to swh/meta$1196

Test Plan

without fix, override container swh-scheduler-runner:

doco up -d
doco logs -f swh-scheduler-runner  # fails to start with paste error

With the fix, the container starts.

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Migrated from D6537 (view on Phabricator)