Remove `--no-cache` for `apk` in Dockerfile

As there is `apk upgrade` and `apk update`, the apk index will already
be existed. `--no-cache` is for `apk` when there is no `apk update`
behavior and it's expected to be no local cache left, not suitable for
the use case here, which wants to upgrade all the package to the latest
when packaging the image.

Dockerfile

@@ -2,7 +2,7 @@ FROM alpine:3.11
 
 RUN apk update && \
     apk upgrade && \
-    apk add --no-cache bash procps drill git coreutils libidn curl && \
+    apk add bash procps drill git coreutils libidn curl && \
     addgroup testssl && \
     adduser -G testssl -g "testssl user"  -s /bin/bash -D testssl && \
     ln -s /home/testssl/testssl.sh /usr/local/bin/ && \