A Dog Named Duke (2012) on Netflix

Plot summary

Terry Polesky served as a sergeant in the Marines. His last tour in Afghanistan left him with shrapnel that required two surgeries, and he was judged medically unfit to return to active duty. In pain and convinced he had become a burden to his schoolteacher wife Bethany and their daughter Alice, he left the family behind. Ten years on he is homeless, sleeping in a mobile trailer parked mostly in his friend Javier's back yard and earning money through a small repair service run from it. Duke, a border collie who followed Bethany home years earlier, shares those years with him. When the dog falls ill and Terry has no money for tests or treatment, he brings Duke to a clinic run by Dr. Angela, leaving only a note.