Add Buildkite pipeline to push Docker image

[pquentin]

Closes #407

Here's a successful build: https://buildkite.com/elastic/eland-release-docker/builds/9#018af52e-7ca0-4e9c-8aa5-6fd9cd4348de. This also reintroduces https://github.com/elastic/eland/pull/586, using a slim Debian 12 base image, which will greatly reduce the noise from vulnerability scanners.

I've left #600 as future work.

[pquentin]

So the lint runs with the latest Python, which as of yesterday is Python 3.12. While we could use Python 3.10 instead (the latest version we support), the lint errors do make sense and the fixes work in Python 3.8+. For the record:

• Python 3.12 has improved f-string support, which apparently allowed flake8 to detect indentation errors in f-strings: e968cd6 (#613)
• Python 3.12 removed the distutils module that we were using to compare versions, this should be done with the third-party packaging model now: 1dbe81d (#613)