Make apk package install more elegant

Dockerfile

@@ -11,9 +11,9 @@ RUN npm install
 RUN npm prune --production
 
 # Install required apk-packages & delete cache
-RUN apk update && apk add tcpdump && rm -rf /var/cache/apk/*
+RUN apk add --no-cache tcpdump
 
 # Bundle app source
 COPY ./src/ .
 
-CMD ["npm", "run", "start"]
+CMD ["npm", "run", "start"]