chore(env): SERVER_HOSTNAMES variable

- With retro-compatibility with `SERVER_HOSTNAME`

Dockerfile

@@ -95,7 +95,7 @@ ENV VPN_SERVICE_PROVIDER=pia \
     SERVER_REGIONS= \
     SERVER_COUNTRIES= \
     SERVER_CITIES= \
-    SERVER_HOSTNAME= \
+    SERVER_HOSTNAMES= \
     # # Mullvad only:
     ISP= \
     OWNED_ONLY=no \

internal/configuration/sources/env/serverselection.go

@@ -42,7 +42,10 @@ func (r *Reader) readServerSelection(vpnProvider, vpnType string) (
 	ss.Cities = lowerAndSplit(citiesCSV)
 
 	ss.ISPs = envToCSV("ISP")
-	ss.Hostnames = envToCSV("SERVER_HOSTNAME")
+
+	_, hostnamesCSV := r.getEnvWithRetro("SERVER_HOSTNAMES", "SERVER_HOSTNAME")
+	ss.Hostnames = lowerAndSplit(hostnamesCSV)
+
 	ss.Names = envToCSV("SERVER_NAME")
 
 	if csv := os.Getenv("SERVER_NUMBER"); csv != "" {