Twoliter fails if `build` is a symlink

[cbgbt]

Problem

If the build directory in a Twoliter workspace is a symlink, Twoliter will fail to execute any of its tools from within Docker/BuildKit.

  0.066 /bin/sh: line 1: /host/build/tools/pipesys: No such file or directory

This is because the Twoliter workspace is mounted as /host in the Docker build context, which is unlikely to also contain the symlink target for /host/build. When twoliter attempts to execute a tool, it will be unable to find it.

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Proposed Solution

• Find an alternative mechanism for injecting Twoliter's "tools" into the build context. Buildkit supports adding additional arbitrary "build contexts" to a given build. The host mount in our Dockerfile only seems to be used as a mechanism for getting Twoliter's tools -- perhaps we could explicitly add a context for the tools which follows symlinks.
• An alternative (though leakier) solution could be to detect that build is a symlink that would "escape" the docker build context and raise an error.