Write output files to a directory relative to Prowler

Write output files (CSV, JSON, etc.) to an `output` directory that is relative to prowler itself, no matter where prowler is invoked from.
Simplify Dockerfile by specifying a WORKDIR
Replace ADD command with the more recommended COPY command
Update README to cover how to run in Docker and access saved reports
Add a .dockerignore file to ignore .git and output directories

This partially addresses #570 - previously, within Docker, Prowler was attempting to write
reports to the root `/` directory in the container, which it did not have permission to do.
Instead, reports are now written to a path relative to Prowler

util/Dockerfile

@@ -9,10 +9,12 @@ RUN addgroup -g ${USERID} ${USERNAME} && \
     pip3 install --upgrade pip && \
     pip install awscli ansi2html boto3 detect-secrets
 
-ADD . /prowler
+WORKDIR /prowler
 
-RUN chown -R prowler /prowler/
+COPY . ./
+
+RUN chown -R prowler .
 
 USER ${USERNAME}
 
-ENTRYPOINT ["/prowler/prowler"]
+ENTRYPOINT ["./prowler"]