Bitcoind to use static RPC authentication cookie from file? I'm running bitcoind 0.20.1 inside a Docker container and would like to access its RPC interface from another container. I have put an authentication cookie into a Docker secret and mapped it to /run/secrets/rpc_cookie. Now when I use -rpccookiefile=/run/secrets/rpc_cookie bitcoind tries to write to that file and fails. Of course, it's read-only! How can I instruct bitcoind to just use the cookie from the file and be done with it?
You can't. The cookie file is for bitcoind to write it's automatically generated credentials to for other software to read. If you want to set the RPC credentials, you need to do so within the bitcoin.conf file using the rpcauth option (or rpcuser and rpcpassword but these are deprecated). These could also be specified on the command line.