Bad encoding in environment variable for admin token can cause InfluxDB v2 startup failure

See https://github.com/influxdata/influxdata-docker/issues/492#issuecomment-855691078. If the environment variable for (at least) the initial admin token has an encoding problem, some piece of our entrypoint script will fail silently after initial setup runs.

Start by reading the entrypoint script and the linked issue comment, then trace how an improperly encoded initial admin-token environment variable is handled after setup. Done means the startup failure is detected rather than silently ignored, with the resulting behavior verified in the repository's existing checks.

Written by the indexing model from the issue text.