**TL;DR**: When upgrading from `Bitcoin Core 0.20.1` to `Bitcoin Core 0.21.0`, a downstream application using the HTTP JSON-RPC API started getting HTTP 403 "Forbidden".

Turns out it is because I have `rpcallowip=::/0` in `bitcoin.conf` which used to work but does not anymore.
As a workaround, `rpcallowip=0.0.0.0/0` works.

-----

I run bitcoind in docker container1, and some application using the HTTP JSON-RPC API of bitcoind is in docker container2. They are on the same "docker network".

In `bitcoin.conf` I have:
```
rpcauth=<...>
rpcbind=0.0.0.0:8332
rpcallowip=::/0
```
(bitcoind RPC is not actually exposed to the public internet; it is up to the docker layer and above to ensure this.)

The application in container2 is connecting through HTTP accessing an URL such as `http://user:pass@172.19.1.10:8332/`.

This setup used to work when running `Bitcoin Core 0.20.1` in container1.

It no longer works with `Bitcoin Core 0.21.0`.

-----

If I change my `bitcoin.conf` as below, it starts working again using 0.21:
```
rpcauth=<...>
rpcbind=0.0.0.0:8332
rpcallowip=0.0.0.0/0
```

So `rpcallowip=::/0` used to work but does not anymore; `rpcallowip=0.0.0.0/0` works.

-----

Easy way to test:
`curl --data-binary '{"jsonrpc":"1.0","id":"curltext","method":"getblockchaininfo","params":[]}' -H 'content-type:text/plain;' http://user:pass@172.19.1.10:8332/`
args.GetArg("-datadir", ""), fs::PathToString(fs::current_path()));
    }
    ValidationCacheSizes validation_cache_sizes{};
    ApplyArgsManOptions(args, validation_cache_sizes);
    if (!InitSignatureCache(validation_cache_sizes.signature_cache_bytes)
        || !InitScriptExecutionCache(validation_cache_sizes.script_execution_cache_bytes))
    {
        return InitError(strprintf(_("Unable to allocate memory for -maxsigcachesize: '%s' MiB"), args.GetIntArg("-maxsigcachesize", DEFAULT_MAX_SIG_CACHE_BYTES >> 20)));
    }
    int script_threads = args.GetIntArg("-par", DEFAULT_SCRIPTCHECK_THREADS);
    if (script_threads <= 0) {
        
        
        script_threads += GetNumCores();
    }
    
    script_threads = std::max(script_threads - 1, 0);
    
    script_threads = std::min(script_threads, MAX_SCRIPTCHECK_THREADS);
    LogPrintf("Script verification uses %d additional threads\n", script_threads);
    if (script_threads >= 1) {
        StartScriptCheckWorkerThreads(script_threads);
    }
    assert(!node.scheduler);
    node.scheduler = std::make_unique<CScheduler>();
    
    node.scheduler->m_service_thread = std::thread(util::TraceThread, "scheduler", [&] { node.scheduler->serviceQueue(); });
    
    node.scheduler->scheduleEvery([]{
        RandAddPeriodic();