Clean up apk cache in Dockerfile after packages installed

This will make the image smaller.

Dockerfile

@@ -3,6 +3,7 @@ FROM alpine:3.11
 RUN apk update && \
     apk upgrade && \
     apk add bash procps drill git coreutils libidn curl && \
+    rm -rf /var/cache/apk/* && \
     addgroup testssl && \
     adduser -G testssl -g "testssl user"  -s /bin/bash -D testssl && \
     ln -s /home/testssl/testssl.sh /usr/local/bin/ && \