Properly upgrade packages and clean up apt cache in Dockerfile (#218)

Result:
```
REPOSITORY     TAG       IMAGE ID       CREATED              SIZE
ssh-audit      after     03e247aee0cc   About a minute ago   131MB
ssh-audit      before    609962ceafb1   About a minute ago   150MB
```

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 && rm -rf /var/lib/apt/lists/*
+RUN apt update && apt -y upgrade && apt -y dist-upgrade && 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