[BUG] Add foreman to docker config described in the README

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Before we start...:

  • I checked the documentation and found no answer
  • I checked to make sure that this issue has not already been filed
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Expected behavior:

Information about how to run dotenv_validator when you use foreman.

Actual behavior:

No informatino about how use dotenv_validator when you use foreman.

Context and environment:

We can update this part of the README: https://github.com/fastruby/dotenv_validator#if-you-use-docker-compose-read-this
to explain that it applies to foreman too.

For more context refer to this issue: fastruby/rails-template#23

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