I'm sorry for the basic question, but...

Please assume I'm a complete moron and explain space docks to me. I get the production capacity of [planet resources] +2, but how does this work when building shops etc???

The production capacity is the number of plastic units you can produce at that space dock each time you have the option of building things. You can spend as much money as you want, but you can't build more plastic than the limit.

When you build GF or FT, that's 2 pieces of plastic for 1 resource. Counts for two against the limit, it's just cheaper. You can build a solo GF if you really want it and the production capacity won't allow 2, but you still pay full price.

When building at space dock there are 3 constraints:

1. You are limited by your resources available and the cost of the unit printed on the Race card. Other additions are refineryif you use theses optional rules or the tech: Sarween tools.

2. You are limited by the number of ships (War suns, Dreadnoughts, Cruisers, Carriers, Destroyers) that can be on that Hex. The limit is set by the number of command counters on your fleet supply.

3. A Space dock can only build a number of plastics (units) = Planet resources(+ any refinery)+2 for Dock+ 1 if you have the Tech: Enviro compensators.

I am not sure what you mean by shops. If building a Space dock in a multi planet system that already has a Space dock this does not count against your production at the existing Space dock because you are building on the empty planet not at the existing Space dock. Refineries and colonies don't count against Space docks production either.

I am not sure what you mean by shops.

I believe that the mentioned "shops" was a typo when writing "ships".