First I'd try soaking the container in hot soapy water for 15-30 minutes. Sometimes the label and adhesive will just slide off after it's soaked.
If that doesn't work, try some Goo Gone, or if you don't have that you can always try WD40 or baby oil. Sometimes it will take some elbow grease to get all the residue off.
As a last resort, you could try heating the label with a hair dryer hopefully peel it off with ease. Just be careful not to melt your container. I've made a few milk jug skulls this year and after running the heat gun over the label, it just peels off. Since I'm assuming you're using a clear plastic container, I don't know if I'd risk a heat gun on it or not.
Hope some of this helps.
