IIRC, tatoos are fine as long as they are not visible when in uniform but do you really want to jeopardize your career for the fanciful and frankly, fantastic, notion that getting some ink impregnated into your skin will complete your life?
The answer must surely be no. Wait till you are in, test the water, then get your tatto, or not.
if you get one now you should be fine. you got to think about healing time tho. bearing in mind your doing physical work that will probably tear the scab off etc. but in the first week you don't do much of that. I'm going back a week tomorrow and was planning on getting mine extended on my arm but think I'll give it a miss due to that cause. But if you get it in the next week you should be fine