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

.gitignore

@@ -21,7 +21,7 @@ tags
 *.DS_Store
 
 # Prowler output
-prowler-output-*
+output/
 
 # JUnit Reports
 junit-reports/