compose: use fixed image tags

Makefile

+images: image-broker
+
+image-broker: image-%: Dockerfile.%
+	docker build -t registry.git.rwth-aachen.de/acs/public/villas/demo/$* -f $<

docker-compose.yml

@@ -9,14 +9,14 @@ volumes:
 services:
   # The VILLASweb frontend
   frontend:
-    image: villas/web-frontend
+    image: villas/web-frontend:demo-v0.1
     build:
       context: frontend
     restart: always
 
   # The VILLASweb backend
   backend:
-    image: villas/web-backend
+    image: villas/web-backend:demo-v0.1
     build:
       context: backend
     environment:
@@ -34,7 +34,7 @@ services:
 
   # A single reverse proxy for all our services
   nginx:
-    image: nginx:stable-alpine
+    image: nginx:1.17.4-alpine
     volumes:
      - ./etc/nginx/villas.conf:/etc/nginx/conf.d/default.conf
     ports:
@@ -47,14 +47,14 @@ services:
 
   # The MongoDB database for the VILLASweb backend
   database:
-    image: mongo:latest
+    image: mongo:3.4.23
     volumes:
       - database:/data/db
     restart: always
 
   # VILLASnode, the gateway between UDP and WebSocket traffic
   node:
-    image: villas/node:latest
+    image: villas/node:demo-v0.1
     privileged: true
     command: node /etc/villas/node/demo.conf
     volumes:
@@ -68,13 +68,13 @@ services:
 
   # A relay for websocket nodes
   relay:
-    image: villas/node
+    image: villas/node:demo-v0.1
     restart: always
     command: relay
 
   # Web Interface for MongoDB
   mongo-express:
-    image: mongo-express
+    image: mongo-express:0.49.0
     environment:
       ME_CONFIG_MONGODB_SERVER: database
       ME_CONFIG_BASICAUTH_USERNAME: admin
@@ -89,6 +89,7 @@ services:
     build:
       context: .
       dockerfile: Dockerfile.broker
+      image: villas/broker:demo-v0.1
     environment:
       RABBITMQ_DEFAULT_USER: "villas"
       RABBITMQ_DEFAULT_PASS: "s3c0sim4!"