You don't fully open the drain. If you do, the air pressure will create a vortex and stop it from draining. Crack it open until it spits and starts coughing up air, then cut it back some until all it's doing is gurgling.
Repeat until you can't get it to gurgle anymore.
Pipe it with some flexible copper to a nearby drain for simplicity.
It's relatively easy to find UV resistant compressor hose from many manufacturers. It's also important to note the "resistant" aspect of the description. Solar UV is quite destructive and I doubt any manufacturer would consider the product to be UV proof.
Encasing your UV resistant hose in a UV opaque tube, conduit, enclosure would substantially reduce the destructive effect of the sun's UV rays. Most compressor hoses are quite durable in all temperatures and you should note that the manufacturer's specifications will include recommended temperatures. Rarely are those figures outside of human habitable levels.
I cannot comment on how to determine if an enclosure selection would be UV opaque, although black colours are usually a better choice than white and other lighter colors. I have personally observed white insulated wiring deteriorate in sunlight over a period of months, while the black insulated equivalent lasted years.
If flexibility is a requirement, one could purchase a (likely expensive) larger diameter UV resistant hose and slide the smaller working compressor hose within.
Even a selection of plumbing grade PVC will provide protection. According to this link of somewhat limited content, a coat of latex paint will increase the UV resistance and lifespan of ordinary PVC pipe.
Additional research shows that one can purchase specifically-UV-resistant PVC, saving the need for a coat of paint!
One more tidbit. Black polyethylene pipe can be flexible enough if needed and is available in many sizes and is quite UV resistant. Using grey PVC risks the pipe being mistaken for electrical conduit (in the USA) and is contra-indicated, despite its UV resistance.
Black PE pipe would never be mistaken for electrical and is also going to be reasonably priced, unless it is a rarity in your country.
Best Answer
As the reservoir for an air compressor holds air under pressure, the forces on the dent will be outward. If the dent is not on a seam or other metal discontinuity, and is shallow enough, little will happen. When it is pressurized the first time, if it does not "un-dent" itself, you likely have little about which to worry. Even if it does pop out, it won't go beyond flush and again, no worry.
Keep an eye on the dent. If the paint is scratched and the metal under begins to rust, not so good. If the paint is cracked or scratched, consider to apply primer and topcoat to remove future rust problems.
Rust is a problem over long periods of time, especially on the inside, so drain your tank frequently and examine it for outside rust and damage as well.