Write attempted config file location into the log

src/main.rs

@@ -1,3 +1,4 @@
+use std::env;
 use std::io::Cursor;
 use std::{fs::File, io::BufReader};
 
@@ -51,7 +52,17 @@ enum RuntimeError {
 
 impl Config {
 	fn load() -> Result<Self, ConfigLoadError> {
-		let file = File::open("config.json").map_err(ConfigLoadError::OpenFailed)?;
+		let env_var_name = "AMPLIFIER_BOT_CONFIG_FILE";
+		let default_filename = "./config.json";
+		let path = env::var(env_var_name).unwrap_or_else(|_err| {
+			warn!(
+				"Cannot read env var '{}', assuming '{}'",
+				env_var_name, default_filename
+			);
+			default_filename.to_owned()
+		});
+		info!("Reading configuration from {}", path);
+		let file = File::open(path).map_err(ConfigLoadError::OpenFailed)?;
 		let reader = BufReader::new(file);
 		serde_json::from_reader(reader).map_err(ConfigLoadError::ReadError)
 	}
@@ -62,7 +73,6 @@ async fn main() -> Result<(), UnresolvableError> {
 	pretty_env_logger::init();
 	info!("Starting amplifier-bot");
 
-	info!("Reading configuration");
 	let config = Config::load().map_err(UnresolvableError::ConfigLoadError)?;
 
 	info!("Long-polling for updates...");