Provide a Docker image with this app

[gdubicki]

Is your feature request related to a problem? Please describe.

We are pushing metrics to Datadog using this app in various CI/CD pipelines. Sometimes we use own images and then we add datadogpy to it, but sometimes the only thing we want from a CI/CD step is to push a metric, so we could use an official Docker image with this tool.

Describe the solution you'd like

Please consider extending https://github.com/DataDog/datadogpy/blob/master/.github/workflows/release.yaml with creating and pushing also a Docker image to https://hub.docker.com/r/datadog. Probably Alpine-based image would be best here for a minimal size.

Describe alternatives you've considered

We can use our own Docker image, but it will either be bigger than it should be or we would have to create and maintain a pipeline to build it ourselves, while probably there would be many other users who would benefit it this would be done officially, here.

[github-actions]

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[gdubicki]

If you are interested I may provide a PR.