Handling multiple glibcs

[zzak]

I think there are a couple ways to avoid the issues described in #37.

Provide multiple glibcs in the runtime

It's entirely possible to build multiple glibc and provide them in a standard place on the system.

Then let the user pick, or override the default, when compiling their binary.

Provide multiple images

Another option is to wrap matsumotory/rcon#4 here.

Right now we only provide one image under the latest tag that is built with Ubuntu 14.04.

We could consider multiple tags for different Linuxes, such as FROM hone/mruby-cli:centos6 could replace FROM hone/mruby-cli in the Dockerfile.

Within the boundries of docker, I'm not sure of another way to accomplish this.