Add basic CI with linters

[birthdaysgift]

We want PRs to have an explicit indicator for linter errors in the branch/PR. It will allow us to see the state of the branch without explicitly running linters locally if we don't want it.

In the scope of this task we need to add a basic CI pipeline with linters and status indicator on GitHub.

Steps to do:

• add Dockerfile for the application (we don't want to add ENTRYPOINT in it for now, since we suppose that it is better to have container entrypoints inside the docker-compose file)
• add docker-compose file
  Compose file should have following services:
• forum123-build (will be used only to build application image, it will allow us to use caching of the app build on CI)
• forum123-mypy (to run mypy)
• forum123-flake8 (to run flake8)
• forum123-pylint (to run pylint)
  We want them to be separated on different services to be able to run build in one job, and then run all linters in a parallel way in three different jobs (parallel execution will be implemented sometimes later, now we need only to have different services in compose file).
• setup CI pipeline using GitHub Actions

Also we want to put this configuration in a separate folder like envs/dev to indicate that this is only for development purposes. And when we will need to add some production infrastructure it will go into envs/prod.
Seems like using different folders is more convenient than having a bunch of Dockerfile's and docker-compose files with suffixes like .dev and .prod.

We decided to put mypy stubs in requirements-dev.txt, because we had a troubles with installing types on CI. In our case we had to run mypy twice - first time, to populate mypy's cache and define what types are missing, and second time to install types and check for errors. The cause of this isssue is that github always run its jobs in a new clean containers, so each new mypy run will not have mypy's cache from the previous run. Therefore we had to run mypy twice for every job to have type stubs installed. More about related problems with missing type stubs on CI from other people you can read here: python/mypy#10600

Worth to mention that now we're not going to make this pipeline optimized by performance. We are planning to add caching of intermediate docker layers later in the scope of another task. For now we just need this pipeline to work and nothing more.

[birthdaysgift]

Here I'll mention some resources which can help us to optimize this pipeline with docker layers caching:

• https://evilmartians.com/chronicles/build-images-on-github-actions-with-docker-layer-caching
• https://testdriven.io/blog/faster-ci-builds-with-docker-cache/#ci-environments
• https://betterprogramming.pub/sharing-cached-layer-between-docker-and-docker-compose-builds-c2e5f751cee4
• https://stackoverflow.com/questions/58592259/how-do-you-enable-buildkit-with-docker-compose
• https://docs.docker.com/compose/compose-file/build/
• Update/Add Docker Compose v2 actions/runner-images#4657
• https://github.com/orgs/community/discussions/25728
• https://levelup.gitconnected.com/github-actions-how-to-share-data-between-jobs-fc1547defc3e
• https://github.com/actions/cache/blob/main/action.yml
• https://docs.docker.com/build/buildkit/

Build app:

• cold cache: 60s
• warm cache: 43s

Next step is improving linters performance using their cache options

• see .mypy_cache for mypy
• see PYLINTHOME for pylint