[AIRFLOW-XXX] Set inside docker env

[milton0825]

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Description

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Certain scripts should only be ran inside a docker container [1]. Setting a flag to indicate whether we are inside the docker container and warn users if they are not running certain scripts in the docker container.

This PR goes hand-in-hand with #5003

[1] #4984

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Code Quality

• Passes flake8

[mik-laj]

CC: @potiuk

[ashb]

@milton0825 Probably combine this with the change to the script in the same PR

[milton0825]

@ashb do we need to rollout this first as we will need to build the airflow image first?

[ashb]

Oh this might not even help - the image we run in is currently airflowci/incubator-airflow-ci:latest - not the one in this repo.

So perhaps the fix is to set this env in scripts/ci/docker-compose.yml, not the Dockerfile

[milton0825]

I see let's push out #5003 then.

Should we remote this Dockerfile since it is not used by CI? I can be a bit confusing to the developer.

[ashb]

This docker file is what apache/airflow on docker hub is, so I think we should keep it :D

[milton0825]

Got it!