Let's say you're staying at a non-Jewish relative's house. He bought you kosher food because he knows you keep kosher. He buys you a drink such as milk. You see him pour the drink for himself into his own non-kosher glass. While pouring, the jug's spout touches his glass. Are you allowed to use that jug to drink out of?
As long as the glass is clean, yes, absolutely. You can even drink from one of his non-kosher glasses.
You can use a non-kosher vessel for kosher food, with the following conditions:
As long as those conditions are met, it doesn't "treif" up your kosher food.