Fix python dependency version for NightHawk Benchmark image

[abaptiste]

When invoking ci/docker/benchmark_build.sh, the image build fails for me with:

fetch https://dl-cdn.alpinelinux.org/alpine/v3.13/main/x86_64/APKINDEX.tar.gz
fetch https://dl-cdn.alpinelinux.org/alpine/v3.13/community/x86_64/APKINDEX.tar.gz
ERROR: unable to select packages:
  python3-3.8.7-r1:
    breaks: world[python3=3.8.7-r0]
The command '/bin/sh -c apk add --no-cache docker=20.10.3-r0 openrc=0.42.1-r19 python3=3.8.7-r0' returned a non-zero code: 1

The change in this PR updates the Dockerfile to ensure that the python version listed is the minimum required version.

With this change in place, the docker image build script completes successfully:

$ ci/docker/benchmark_build.sh
...
Step 8/11 : RUN apk add --no-cache docker=20.10.3-r0 openrc=0.42.1-r19 python3>=3.8.7-r0
 ---> Running in 88d67905ec33
...
 ---> 10194209eab7
Successfully built 10194209eab7
Successfully tagged envoyproxy/nighthawk-benchmark-dev:latest
docker build finished

cc: @oschaaf

Signed-off-by: abaptiste abaptiste@users.noreply.github.com

[mum4k]

Thank you for the fix @abaptiste.