Moisture inside is a physical element of atmosphere....my definition. As a body (your container for one) sits out in the weather, changes bring on humid and dry conditions. Humidity is sucked in when the container cools during the night. The next day the container gets hot....but not that hot, and the air expands inside and "exhales"....but it exhales dry air. Next evening it cools and inhales more humid air. Before long you have sweating.
My advice would be to figure out a way for the container to breathe. Whirling roof vents are one answer with pickup vents somewhere along the sides. Think about attics in houses as an example...gable ends have peak of the roof vents. Hipped have whirly birds, flat covered square vents near the peak, or peak vents with input vents under the eaves. Attics that aren't vented will mildew. Vented won't. Your steel container is much more susceptible due to being steel (higher cycling temp due to material), not insulated, and closed. That's the way I see it.