In a Google Docs document, I would like to be able to specify the page margins (File → Page Setup) in millimeters instead of inches.
Is it possible to switch between imperial and metric units?
Go to Settings → Document Settings on the top right of the page and change the language to English (UK).
Rulers and margins will now both be in centimetres (cm).
You may need to log out and sign back in before you see the changes take effect.
This is not possible in the current version of Google Docs with "English (US)" regional settings. Until they change this you can convert millimeters to inches using this Google query:
Or you could follow the tips in Dominiks answer, and change your language settings to "English (UK)".