Would you consider it safe to clean and reuse a 20 litre plastic drum that contained 36% Hydrochloric Acid for drinking water? If so how would you recommend cleaning it?
Regardless of what it takes to remove any traces of hydrochloric acid from the drum, the container is probably not certified as food-grade. So in addition to any risks from the acid itself (including things like heavy metals, which are common contaminants in mineral acids), the plastic is not certified to be free of toxic substances left over from manufacturing or that the plastic will not degrade to release such substances over its lifetime.
I imagine the container is made of polyethylene or polypropylene (it should be marked with a numeric code). If that is the case, I have a hard time accepting that there is any downside to using it for drinking water. Chemists don't want heavy metals contaminating their hydrochloric acid, so I think you are OK on that count as well.