Can I compile a sketch for my attiny85 without an arduino board?
Why wouldn't you be able to? 

The compiler runs on the computer, not on the Arduino board. You don't need a board to be connected to compile either. If you enable verbose output during compile in the Arduino IDE settings it will tell you where it puts the resulting .hex file, after which you can use avr-dude to upload it to the target MCU.

If you want to use Arduino libraries you'll need to use an core for ATTiny devices (e.g. ATTinyCore). But either way you don't need an Arduino board to compile.



I'd also echo what @Marcus says, for the small devices you are better off completely ditching the Arduino stuff (it's such badly written, bloated, and inefficient), and simply starting with a nice clean C file.

There is nothing stopping you using the Arduino IDE or the avr-libc tools which it uses. If you add your own main() function in a C file and don't put anything in the .ino file you will be able to compile without any of the bloat.