Rain falls on the greenhouse, runs into a water butt, from the tap on the butt runs a pipe into the greenhouse into a very small cistern with a ball valve, from the cistern pipes run into capped end rain gutters so to keep a 2" deep level. The gutter is screwed between two 6x2 beams that take the weight of the tubs . The tubs need a hole cut into the bottom using a tank cutter, into the hole you place small water plant pots. When you fill the tubs with soil fill the water plant pot as this wicks up the water into the tub ,fill the tub completely and replace the lid ( lid has a similar sized hole as the tub) and plant through this hole, being covered it helps to stop evaporation. Also add a fitting at top top of the water butt so it can be filled if it does not rain.
If you want photos I can do that day after tomorrow as I'm going fishing tomorrow.