Use genesis coinbase address for ckpool startup self-test

ckpool-solo validates conf btcaddress once at startup to prime the
coinbase-builder, then pays solves to the worker's stratum address.
The conf value never sees a satoshi, so use the active network's
genesis coinbase address instead of a burn placeholder.

docker_entrypoint.sh

@@ -21,10 +21,16 @@ case "${BITCOIND_VARIANT}" in
     bitcoind)
         export BITCOIN_RPC_HOST="bitcoind.embassy"
         export BITCOIN_RPC_PORT=8332
+        # Satoshi's genesis block coinbase address. Used only for
+        # ckpool's startup coinbase-builder self-test; never credited
+        # a satoshi since solo mode pays the worker's stratum address.
+        SELFTEST_ADDRESS="1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa"
         ;;
     bitcoind-testnet)
         export BITCOIN_RPC_HOST="bitcoind-testnet.embassy"
         export BITCOIN_RPC_PORT=48332
+        # Testnet genesis coinbase address — valid P2PKH on testnet4.
+        SELFTEST_ADDRESS="mipcBbFg9gMiCh81Kj8tqqdgoZub1ZJRfn"
         ;;
     *)
         echo "kamado-entrypoint: unknown bitcoind variant: ${BITCOIND_VARIANT}" >&2
@@ -46,10 +52,15 @@ TLS_ENABLED=$(q '.tls.enabled // "disabled"')
 TLS_PORT=$(q '.tls.port // 3334')
 
 # CKPool-solo uses the worker's stratum username as the payout
-# address. Kamado refuses to authenticate workers whose username is
-# not a valid BTC address, so the conf `btcaddress` fallback is never
-# reached. We still set it to a placeholder the conf parser accepts.
-export POOL_BTCADDRESS="1BitcoinEaterAddressDontSendf59kuE"
+# address and refuses to authenticate workers whose username is not
+# a valid address on the active network. The conf `btcaddress` is
+# only consulted once at startup, for ckpool's coinbase-builder
+# self-test: it builds and validates a sample coinbase transaction
+# against bitcoind before accepting any workers. Since no worker has
+# connected yet at that point, we hand it the genesis block coinbase
+# address for the active network — it's always valid, and solo mode
+# never credits it a satoshi.
+export POOL_BTCADDRESS="${SELFTEST_ADDRESS}"
 
 export LOGDIR=/var/log/ckpool
 export SOCKET_DIR=/run/ckpool