[#10461] Add docs about custom environment
minor #10470 [server:run] [#10461] Add docs about custom environment (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
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[server:run] [#10461] Add docs about custom environment

| Q             | A
| ------------- | ---
| Fixed tickets | #10461, https://github.com/symfony/symfony-docs/issues/3688
| License       | MIT

It's better to document this specific usecase in the command help.

Commits
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6871a61 [#10461] Add docs about custom environment