Add cleanup for apt cache files (#215)

* Add cleanup for apt cache files

Adding this command decreases the size of the image.

ssh-audit-new                                           latest                 0c391ba567ee   39 minutes ago   157MB
ssh-audit-old                                           latest                 a425e0043125   40 minutes ago   176MB

* Fix Dockerfile

Forgot to add logical "and" (&&)

Dockerfile

@@ -3,7 +3,7 @@ FROM python:3-slim
 WORKDIR /
 
 # Update the image to remediate any vulnerabilities.
-RUN apt clean && apt update && apt -y dist-upgrade && apt clean
+RUN apt clean && apt update && apt -y dist-upgrade && apt clean && rm -rf /var/lib/apt/lists/*
 
 # Remove suid & sgid bits from all files.
 RUN find / -xdev -perm /6000 -exec chmod ug-s {} \; 2> /dev/null || true