Make signerUrl an environment variable with default fallback

[bh2smith]

Have been running into the following issue in the keystore UI

Web3Signer is not available. Its API is not responsive. Check if the Web3Signer package is running. To avoid slashing, do not upload your validator keystores to another machine.

with internal logs in the brain as follows:

dappnode/DAppNodePackage-web3signer-gnosis#102

[DEBUG] Received rpc call: signerGetStatus
[ERROR] SignerApiError: Error getting (GET) server status. Is Web3Signer running?: Request to /healthcheck failed with status code ENOTFOUND: getaddrinfo ENOTFOUND signer.gnosis.dncore.dappnode. 
SignerApiError: Error getting (GET) server status. Is Web3Signer running?: Request to /healthcheck failed with status code ENOTFOUND: getaddrinfo ENOTFOUND signer.gnosis.dncore.dappnode. 
    at new SignerApiError (file:///app/packages/brain/dist/modules/apiClients/signer/error.js:4:9)
    at Web3SignerApi.getStatus (file:///app/packages/brain/dist/modules/apiClients/signer/index.js:148:19)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async signerGetStatus (file:///app/packages/brain/dist/modules/apiServers/ui/calls/signerGetStatus.js:6:12)
    at async Object.signerGetStatus (file:///app/packages/brain/dist/modules/apiServers/ui/calls/index.js:41:38)
    at async Socket.<anonymous> (file:///app/packages/brain/dist/modules/apiServers/ui/startUiServer.js:47:36)

These logs suggest that the brain is looking for the API at signer.gnosis.dncore.dappnode (which does not resolve). It is pretty clear that the correct URL is actually web3signer.web3signer-gnosis.dappnode

Currently there is no way to override this value (neither in UI or shell).

Would be nice if we could set it.

StakingBrain/packages/brain/src/modules/config/index.ts

Line 55 in 48464dc

signerUrl: `http://signer.${network}.dncore.dappnode:9000`,