Initialize unset git user.email / user.name correctly; fix Dockerfile

Previously if user.name was set but user.email wasn't, the default value for user.email would not be set. This caused errors for the provided dockerfile (as reported in issue #328 but not properly fixed).

Either of these commits fixes the issue for running in a container, but I felt it was sane to fix the root cause (defaults not being applied to both of them necessarily) and the Dockerfile for sanity's sake (e.g. if someone wanted to docker exec a git command before gogs initted the user name, this would fix that too).

I verified that after my config change if I did not set a user.email I got the following message "[I] Git config user.email set to gogitservice@gmail.com" and was able to initialize a repository with an empty readme successfully.