Update the sbcl version and tweak the apt-get command line

I've updated sbcl to 1.2.5.
The image was also reporting that it couldn't find wget. I replaced the ; separators on the apt-get line with && conjunctions, and now it builds happily.
I also added a `cd /tmp` step to get out of the directory that we then delete.

Thanks for creating this image; I hope you find these tweaks useful.