fix docker test and dissallow sending dex fee

mm2src/coins/utxo/utxo_common.rs

@@ -3991,6 +3991,14 @@ pub async fn get_fee_to_send_taker_fee<T>(
 where
     T: MarketCoinOps + UtxoCommonOps,
 {
+    if DexFee::Zero == dex_fee {
+        return Ok(TradeFee {
+            coin: coin.ticker().to_owned(),
+            amount: MmNumber::default(),
+            paid_from_trading_vol: false,
+        });
+    }
+
     let decimals = coin.as_ref().decimals;
 
     let outputs = generate_taker_fee_tx_outputs(decimals, &AddressHashEnum::default_address_hash(), &dex_fee)?;

mm2src/mm2_main/src/lp_swap/maker_swap.rs

@@ -739,6 +739,19 @@ impl MakerSwap {
         };
         swap_events.push(MakerSwapEvent::MakerPaymentInstructionsReceived(instructions));
 
+        let taker_amount = MmNumber::from(self.taker_amount.clone());
+        let dex_fee = dex_fee_amount_from_taker_coin(self.taker_coin.deref(), &self.r().data.maker_coin, &taker_amount);
+
+        if coins::DexFee::Zero == dex_fee {
+            info!("Zero Dex Fee: Skipping taker fee");
+            let fee_ident = TransactionIdentifier {
+                tx_hex: BytesJson::from(vec![]),
+                tx_hash: BytesJson::from(vec![]),
+            };
+            swap_events.push(MakerSwapEvent::TakerFeeValidated(fee_ident));
+            return Ok((Some(MakerSwapCommand::SendPayment), swap_events));
+        }
+
         let taker_fee = match self.taker_coin.tx_enum_from_bytes(payload.data()) {
             Ok(tx) => tx,
             Err(e) => {
@@ -751,8 +764,6 @@ impl MakerSwap {
         let hash = taker_fee.tx_hash_as_bytes();
         info!("Taker fee tx {:02x}", hash);
 
-        let taker_amount = MmNumber::from(self.taker_amount.clone());
-        let dex_fee = dex_fee_amount_from_taker_coin(self.taker_coin.deref(), &self.r().data.maker_coin, &taker_amount);
         let other_taker_coin_htlc_pub = self.r().other_taker_coin_htlc_pub;
         let taker_coin_start_block = self.r().data.taker_coin_start_block;
 

mm2src/mm2_main/tests/docker_tests/docker_tests_inner.rs

@@ -2167,9 +2167,9 @@ fn test_get_max_taker_vol_with_kmd() {
     .unwrap();
     assert!(rc.0.is_success(), "!max_taker_vol: {}", rc.1);
     let json: Json = serde_json::from_str(&rc.1).unwrap();
-    // the result of equation x + x * 9 / 7770 + 0.0002 = 1
-    assert_eq!(json["result"]["numer"], Json::from("1294741"));
-    assert_eq!(json["result"]["denom"], Json::from("1296500"));
+    // the result of equation x + 0.0002 = 1 (no dex fee included)
+    assert_eq!(json["result"]["numer"], Json::from("99989"));
+    assert_eq!(json["result"]["denom"], Json::from("100000"));
 
     let rc = block_on(mm_alice.rpc(&json!({
         "userpass": mm_alice.userpass,