Try using a systemic weedkiller such as Tumbleweed or SBK brushwood killer, and both contain glyphosate. They are absorbed into the plant and 'kill from within'!! Over large areas it is better to use a watering can, but on individual plants a spray bottle is more economical. Thoroughly rinse the can of bottle after use, or keep seperate ones for weedkiller.
If the dock is persistent you may need to apply it a second or third time!
Use these weedkillers carefully though as they also kill the wanted plants too. You know what's a weed but the weedkiller doesn't!! Lost a few plants myself this way, unfortuntely!!