" So do not corrupt yourselfs by becoming an idol or image of any shape, whether male or female "......................Bo dybuilding 
Bodybuilding itself isn't sinful, but you can quickly introduce elements into that can make it so (ex: illegal drug use/addiction, adultery, homosexuality).
Further, if you worship only at the narcissistic alter of yourself I can easily see bodybuilding becoming idolatrous as it takes the focus off of God.
Philippians 4:13
13 For I can do everything through Christ, who gives me strength.
Hebrews 12:11-12
11 No discipline is enjoyable while it is happening—it’s painful! But afterward there will be a peaceful harvest of right living for those who are trained in this way. 12 So take a new grip with your tired hands and strengthen your weak knees.
1 Corinthians 9:27
27 I discipline my body like an athlete, training it to do what it should. Otherwise, I fear that after preaching to others I myself might be disqualified.
1 Timothy 4:8
8 Physical training is good, but training for godliness is much better, promising benefits in this life and in the life to come.
