Clean up script in container self-test

The current test:

- pa11y-ci --json > pa11y/pa11y-ci-results.json || if [ "$?" -eq "1" ]; then exit 1; else exit 0; fi
- exit 2
 

...could be re-written as follows for consistency and succinctness

- pa11y-ci --json > pa11y/pa11y-ci-results.json && exit 2 || if [ "$?" -eq "1" ]; then exit 1; else exit 0; fi