chore(env): SERVER_CITIES variable

- With retro-compatibility with `CITY`

Dockerfile

@@ -94,7 +94,7 @@ ENV VPN_SERVICE_PROVIDER=pia \
     # VPN server filtering
     REGION= \
     SERVER_COUNTRIES= \
-    CITY= \
+    SERVER_CITIES= \
     SERVER_HOSTNAME= \
     # # Mullvad only:
     ISP= \

internal/configuration/sources/env/serverselection.go

@@ -36,7 +36,10 @@ func (r *Reader) readServerSelection(vpnProvider, vpnType string) (
 	}
 
 	ss.Regions = envToCSV("REGION")
-	ss.Cities = envToCSV("CITY")
+
+	_, citiesCSV := r.getEnvWithRetro("SERVER_CITIES", "CITY")
+	ss.Cities = lowerAndSplit(citiesCSV)
+
 	ss.ISPs = envToCSV("ISP")
 	ss.Hostnames = envToCSV("SERVER_HOSTNAME")
 	ss.Names = envToCSV("SERVER_NAME")