Switch Docker base image from CentOS (as it is now EOL) (close #283)

[cpnat]

As CentOS is now EOL, this change switches the Docker base image (used for development and CI) to debian:bullseye-slim

Debian is actively maintained and receives feature and security updates.

[snowplowcla]

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[cpnat]

Thanks for your pull request. Is this your first contribution to a Snowplow open source project? Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

CLA signed

[coveralls]

Coverage remained the same at 97.007% when pulling 43a84d9 on cpnat:issue/283 into 4e3a4b8 on snowplow:master.

[Jack-Keene]

Hi @cpnat, thanks for your contribution to the Snowplow Python tracker. I have approved this PR and it will be released in the 0.12 version.