entrypoint.sh

#!/bin/bash

# Start the run once job.
echo "Docker container has been started"

# Setup a cron schedule
echo "* * * * * /run.sh >> /var/log/cron.log 2>&1
# This extra line makes it a valid cron" > scheduler.txt

crontab scheduler.txt
cron -f

It looks like it misses the " on line 7. And I think the fixed snippets are as follows:

echo "* * * * * /run.sh" >> /var/log/cron.log 2>&1

I think the line 7 and line 8 are on the same line. And the snippet should look like:

#!/bin/bash

# Start the run once job.
echo "Docker container has been started"

# Setup a cron schedule
printf "* * * * * /run.sh >> /var/log/cron.log 2>&1\n# This extra line makes it a valid cron" > scheduler.txt

crontab scheduler.txt
cron -f

@peter279k Yes, it can be rewritten in a single line as you suggested. The only difference is the former one serves better clarity.