**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/`
if (chainman.m_blockman.IsPruneMode()) {
        if (!fReindex) {
            LOCK(cs_main);
            for (Chainstate* chainstate : chainman.GetAll()) {
                uiInterface.InitMessage(_("Pruning blockstore…").translated);
                chainstate->PruneAndFlush();
            }
        }
    } else {
        LogPrintf("Setting NODE_NETWORK on non-prune mode\n");
        nLocalServices = ServiceFlags(nLocalServices | NODE_NETWORK);
    }
     ********************************************************* Step 11: impor
    if (!CheckDiskSpace(args.GetDataDirNet())) {
        InitError(strprintf(_("Error: Disk space is low for %s"), fs::quoted(fs::PathToString(args.GetDataDirNet()))));
        return false;
    }
    if (!CheckDiskSpace(args.GetBlocksDirPath())) {
        InitError(strprintf(_("Error: Disk space is low for %s"), fs::quoted(fs::PathToString(args.GetBlocksDirPath()))));
        return false;
    }
    int chain_active_height = WITH_LOCK(cs_main, return chainman.ActiveChain().Height());
     On first startup, warn on low block storage space
    if (!fReindex && !fReindexChainState && chain_active_height <= 1) {
        uint64_t assumed_chain_bytes{chainparams.AssumedBlockchainSize() * 1024 * 1024 * 1024};
        uint64_t additional_bytes_needed{
            chainman.m_blockman.IsPruneMode() ?
                std::min(chainman.m_blockman.GetPruneTarget(), assumed_chain_bytes) :
                assumed_chain_bytes};
        if (!CheckDiskSpace(args.GetBlocksDirPath(), additional_bytes_needed)) {