How can I completely clean an old candle container?
I've done similar things after using food containers for candles.  You may not actually need all these steps, but this is how to be very thorough.
What I'd do is put them in a cool oven, just hot enough to thoroughly melt the wax (100°C should be enough).
First, put them face down supported on wires over a drip tray.  The vast majority of the wax will run out.
Then, still hot, and holding using oven gloves, wipe the insides thoroughly with a dry rag.
In terms of food safety, while I doubt you could ever reuse the containers commercially within the rules, I'd probably be happy at that point to use containers from  unscented candles.  After all, paraffin wax and beeswax are both used in food.
I'd still go a step further though, and definitely if anything scented had been in there: Finally, put them through a dishwasher on a hot wash (70°C or above).  Paraffin wax and beeswax both melt a little below this, and once melted the dishwasher will remove the residue and any scent.  Shifting the scent may actually be the tricky bit.
Apparently container candles are best made with lower melting point waxes anyway, so they should easily melt in the dishwasher.  The earlier steps were more to protect your filters. If any scent lingers after dishwashing, either give up on that container, or wipe round with cooking oil, then fill to the brim with hot soapy water.  Dishwash again after soaking.
Of course, there's no guarantee that the containers can withstand a dishwasher.  Paint (or glitter etc.) on the outside would be particularly likely to be damaged.