iCalendar file validator command-line application available as a Docker image.
Run the validator like this:
docker run --volume=<local dir>:/data faph/icalendar-validator /data/calendar.ics
where <local dir> is a directory containing the file calendar.ics.
Based on the ical4j library.
Source code licensed under the MIT License.
ical4j library copyright Ben Fortuna, licensed under the BSD 3-Clause License.