[Snyk] Upgrade postcss from 8.2.15 to 8.3.6

Snyk has created this PR to upgrade postcss from 8.2.15 to 8.3.6.

:information_source: Keep your dependencies up-to-date. This makes it easier to fix existing vulnerabilities and to more quickly identify and fix newly disclosed vulnerabilities when they affect your project.

The recommended version is 7 versions ahead of your current version.
The recommended version was released 2 months ago, on 2021-07-21.

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nickjj / docker-rails-example

[Snyk] Upgrade postcss from 8.2.15 to 8.3.6 #21

Snyk has created this PR to upgrade postcss from 8.2.15 to 8.3.6.