Clean up CI config and Docker image

[dataders]

• CircleCI config is now:
  • greatly cleaned up, and
  • installed the package and deps inline and no longer conflicts with version in Docker container.
• Docker image for CI now:
  • no longer installs the dev_requirements.txt or dbt-sqlserver
  • uses cimg/python:3.7.10 instead of docker/python:3.7-slim as an upstream image.
  • is greatly simplfied due to reliance on CircleCI's official image

fixes #107

[dataders]

@JCZuurmond I'm almost there! Just having issues installing dbt-sqlserver outside of the virtual environment. I think I need to explicitly install and upgrade pip inside the docker image...

[JCZuurmond]

I think the struggles with installing dbt-sqlserver occur because python is installed in the docker image. I think moving the python part out of the docker file should solve this

[dataders]

@JCZuurmond ok now things are looking much cleaner IMHO. what do you think?

[JCZuurmond]

yes, looks good @swanderz! I think it is ready to be merged.

There is one change I suggested before, but do not mind too much if not implemented, which I think is nice: use the cimg/base image in the Dockerfile and add cimg/python as secondary image in the .circleci/config.yml. This makes it more clear in the CI which python version is expected.

[mikaelene]

Looks great!