apk-install on 1 line and delete cache when done

Dockerfile

@@ -7,8 +7,8 @@ WORKDIR /usr/src/app
 COPY package*.json ./
 RUN npm install
 
-RUN apk update
+# Install required apk-packages & delete cache
-RUN apk add tcpdump
+RUN apk update && apk add tcpdump && rm -rf /var/cache/apk/*
 
 # Bundle app source
 COPY ./src/ .