Add docs about http_proxy

docs/src/content/hardhat-runner/docs/troubleshooting/common-problems.md

@@ -13,3 +13,9 @@ npx hardhat --max-memory 4096 compile
 ```
 
 If you find yourself using this all the time, you can set it with an environment variable in your `.bashrc` (if using bash) or `.zshrc` (if using zsh): `export HARDHAT_MAX_MEMORY=4096`.
+
+## Using Hardhat with a proxy server
+
+Hardhat supports the `http_proxy` environment variable. When this variable is set, Hardhat will send its requests through the given proxy for things like JSON-RPC requests, mainnet forking and downloading compilers.
+
+There's also support for the `no_proxy` variable, which accepts a comma separated list of hosts or `"*"`. Any host included in this list will not be proxied. Note that requests to `"localhost"` or `"127.0.0.1"` are never proxied.