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When the temps are hotter the rosebushes will typically shed some foliage or just have less in an effort to keep cooler. Once the cooler weather returns they will likely get bushier.

If you notice some webbing within the foliage like spiders do, it could be spider mites. It could be other small insects as well. I recommend getting some Bayer Tree and Shrub granules and apply it as directed on the label for container grown roses, ornamental shrubs. If only the lowest leaves are effected, it is easier for soils dwelling insects to attack them even in pots/containers.

Here is a link to any article for you: https://www.gardeningknowhow.com/ornamental/flowers/roses/leaves-falling-off-roses.htm . I would also recommend buying some Bayer Tree and Shrub Granules in the big blue container. Apply this product to each of the roses as directed on the label. It moves up and throughout the rosebushes and has taken care of such things as the insects and worms that eat away the under part of the foliage leaving the lace look. I am not sure where you live now but if you climate is hotter, it can bring about new challenges in the garden just as moving from a dry climate to a humid/moist climate can do.

Yes they can be grown successfully in pots. Make sure you get large pots that have drainage holes in them as they do not like overly wet soils that stay too wet. The amount they should be watered depends on the climate where you are. Hotter days will make watering possibly an every day matter. Being it pots it is easy for them to get too dry much more quickly. I recommend buying a moisture meter and check the soils moisture frequently.