Use tagged docker container for synapse

Makefile

 FILES=`find src tests examples -type f -type f \( -iname "*.cpp" -o -iname "*.hpp" \)`
 
+SYNAPSE_IMAGE="avhost/docker-matrix:v0.26.0"
+
 debug:
 	@cmake -GNinja -H. -Bbuild -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=1 -DOPENSSL_ROOT_DIR=/usr/local/opt/openssl
 	@cmake --build build
@@ -14,11 +16,11 @@ test:
 
 synapse:
 	@docker run -v `pwd`/data:/data --rm \
-		-e SERVER_NAME=localhost -e REPORT_STATS=no avhost/docker-matrix generate
+		-e SERVER_NAME=localhost -e REPORT_STATS=no ${SYNAPSE_IMAGE} generate
 	@docker run -d \
 		--name synapse \
 		-p 443:8448 -p 80:8008 -p 3478:3478 \
-		-v `pwd`/data:/data avhost/docker-matrix start
+		-v `pwd`/data:/data ${SYNAPSE_IMAGE} start
 	@echo Waiting for synapse to start...
 	@sleep 5
 	@echo Register alice