How can I remove a smell of laundry detergent of a plastic container?

try white vinegar

I would use vinegar. fill it with vinegar, let it sit a little while and empty.

Try putting vinegar in the container. Let it set a while, then wash well. Put it outside in the sun a day to dry.

vingegar

Wash it out with vinegar water. Try letting vinegar set in it overnight. I have even stuffed containers with newspaper to remove odors.

Andwer

Once plastic gets a smell, it retains the smell. Try using hot vinegar in the bottle, close the bottle and then let it sit for 48 hours. Then empty the container, rinse with hot water and let sit for another 24 hours.