[fargate] [request]: document HOSTNAME environment variable behavior

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It seems in the last few days, Fargate behavior changed, to where you cannot write the `HOSTNAME` environment variable through a task definition. Instead, it's overridden to the actual host's hostname.

For anyone who relied on that variable, it was a breaking change :)

I didn't see it in the environment variables available docs, so that should probably be called out.

**Which service(s) is this request for?**
Only tested this on Fargate

**Are you currently working around this issue?**
Changed envvar name.

Start by locating the Fargate environment-variable documentation and verify the reported HOSTNAME behavior against a task definition. Done means the relevant documentation explicitly explains that HOSTNAME is overridden and identifies the supported workaround or limitation.

Written by the indexing model from the issue text.