Use a specific commit of MQTT-SN

Dockerfile

@@ -1,7 +1,8 @@
 FROM ubuntu:18.04 AS build-mqtt
 
 ENV MQTT_SN_REPO=paho.mqtt-sn.embedded-c
-ENV MQTT_SN_ZIP=https://github.com/eclipse/"$MQTT_SN_REPO"/archive/master.zip
+ENV MQTT_SN_COMMIT=f2dcda358f21e264de57b47b00ab6165bab4da18
+ENV MQTT_SN_ZIP=https://github.com/eclipse/"$MQTT_SN_REPO"/archive/"$MQTT_SN_COMMIT".zip
 
 RUN DEBIAN_FRONTEND=noninteractive apt -y update
 RUN DEBIAN_FRONTEND=noninteractive apt -y install wget unzip libssl-dev build-essential
@@ -10,12 +11,14 @@ RUN DEBIAN_FRONTEND=noninteractive apt -y install libc6-dev-amd64-cross cmake
 RUN wget --progress=dot:giga --no-check-certificate -O "$MQTT_SN_REPO".zip "$MQTT_SN_ZIP"
 RUN unzip "$MQTT_SN_REPO".zip
 RUN rm "$MQTT_SN_REPO".zip
-RUN cd "$MQTT_SN_REPO"-master/MQTTSNGateway && ./build.sh udp6
+RUN cd "$MQTT_SN_REPO"-f2dcda358f21e264de57b47b00ab6165bab4da18/MQTTSNGateway && ./build.sh udp6
 
 FROM openthread/otbr:latest
 
-COPY --from=build-mqtt /paho.mqtt-sn.embedded-c-master/MQTTSNGateway/bin/MQTT* /app/
-COPY --from=build-mqtt /paho.mqtt-sn.embedded-c-master/MQTTSNGateway/bin/*.conf /app/
-COPY --from=build-mqtt /paho.mqtt-sn.embedded-c-master/build.gateway/MQTTSNPacket/src/libMQTTSNPacket.so /usr/local/lib/
+ENV MQTT_SN_COMMIT=f2dcda358f21e264de57b47b00ab6165bab4da18
+
+COPY --from=build-mqtt /paho.mqtt-sn.embedded-c-"$MQTT_SN_COMMIT"/MQTTSNGateway/bin/MQTT* /app/
+COPY --from=build-mqtt /paho.mqtt-sn.embedded-c-"$MQTT_SN_COMMIT"/MQTTSNGateway/bin/*.conf /app/
+COPY --from=build-mqtt /paho.mqtt-sn.embedded-c-"$MQTT_SN_COMMIT"/build.gateway/MQTTSNPacket/src/libMQTTSNPacket.so /usr/local/lib/
 
 ADD docker_entrypoint.sh /app/etc/docker