docker镜像版本需要更新

当前镜像版本的gitit版本还是比较旧的，不能正确的支持gitit-bigger相关配置。自己编译的0.12.2.1版本能够正常使用。
```
root@0854d5f4269c:/data/gitit# gitit --version
gitit version 0.10.0.1 +plugins
Copyright (C) 2008 John MacFarlane
This is free software; see the source for copying conditions.  There is no
warranty, not even for merchantability or fitness for a particular purpose
```