Update .gitignore to ignore local files use for plugin builds

[Domoninic]

add .env & Dockerfile-Plugins so user who build images with plugins don't get these local files tracked

Related Issue: N/A

New Behavior

Local file used for Plugin builds will be ignored by git

Contrast to Current Behavior

When using plugins you need build with a Dockerfile-Plugins and often a .env file. Currently git sees those files a un-tracked and in IDEs (e.g. VSCode ) they show as changes

Discussion: Benefits and Drawbacks

Remove annoyance of having local only file tracked by git and eliminate possibility of accidentally committing them.

Changes to the Wiki

None

Proposed Release Note Entry

Add .env and Dockerfile-Plugins to .gitignore.

Double Check

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• [ x ] I have explained my PR according to the information in the comments.
• [ x ] My PR targets the develop branch.