Investigate using ccache in the Travis build toolchain

[lisa]

It would be nice to use ccache in the "Install musl-cross" build step. Unfortunately there's no support for a bind mount in docker build (in fact, previously I used Rockerfile to accomplish it, but a native Docker solution would be nice since those features are deprecated). Maybe that looks like a thin wrapper to use the Travis CI ccache support? I'll have a look and see what the options are.

[lisa]

grammarly/rocker#199 gives some alternatives to Rocker, however, it doesn't seem to answer this kind of use case where I want to tote along a ccache mountpoint with each build (to say nothing of how that mountpoint would be persisted with Travis-CI).

I think that this will be undoable without some weird convoluted dance (like: curl a tarball down, put it in place, use it, tar it up, put the tarball someplace) that will add time and monetary costs to this build. Sadly, I think this isn't feasible right now.