Make docker container smaller

[thomasmulvaney]

The Pixie VM is small. It would be nice to if the docker image reflected this. Maybe we could use the Alpine base image which is smaller.

[oholiab]

What does the sid image weigh in at? I'm building on alpine right now and it's coming in at a pretty hefty >300MB... I think it's the boost-dev dependencies (pulling in all of the built boost libraries) which might be possible to reduce down.

[thomasmulvaney]

The current image is 1gb apparently: https://imagelayers.io/?images=pixielang%2Fpixie:latest The sid base image is 150mb or so. It looks like the apt-get installing of stuff seems to be the real bulk of it. 300mb is a big improvement. We can probably delete the externals dir after building too. That should wipe of 100mb or so.