add ETHERSCAN_API_URL env to allow customizing the api endpoint per network

.env.example

@@ -1,6 +1,6 @@
 SERVER_PORT=3000
 SERVER_HOST=127.0.0.1
-
+ETHERSCAN_API_URL="https://api.etherscan.io"
 ETHERSCAN_API_KEY=
 
 # DB Statediff

README.md

@@ -25,6 +25,7 @@ And then open in browser `http://172.17.0.2:3000/metrics`
 ```
 docker run \
     -e SERVER_HOST=0.0.0.0 \
+    -e ETHERSCAN_API_KEY="https://api.etherscan.io" \
     -e ETHERSCAN_API_KEY=*** \
     -e STATEDIFF_PG_HOST=*** \
     -d vulcanize/eth-state-metrics

src/etherscan.js

@@ -2,7 +2,7 @@ const axios = require('axios');
 const AppError = require('./error');
 
 const etherscanClient = axios.create({
-    baseURL: 'https://api.etherscan.io',
+    baseURL: process.env.ETHERSCAN_API_URL,
 });