Workflow for regenerating par files is broken

[brandjon]

Running the update_tools.sh script within the docker contain leads to an error. First there's an incompatible change issue

sequence element must be a string (got 'path'). See bazelbuild/bazel#7802 for information about --incompatible_string_join_requires_strings.

but this is a red herring, since it occurs within the control flow of reporting another error. Bypassing it by adding --incompatible_string_join_requires_strings=false on the bazel command line in the Dockerfile, we get

java.io.IOException: Cannot run program "git" (in directory "/root/.cache/bazel/_bazel_root/eb88a7e7a2050c89c11b77b84df21520/external/io_bazel_rules_sass"): error=2, No such file or directory

So it appears that we need to ensure git is available within the docker container.

This is blocking #203, which itself blocks #202.

[brandjon]

#210 proposes to pin the bazel dependency within the docker image when regenerating the tools. This sounds like the right idea, in addition to the fix to be compatible with a newer bazel.

The docker image is also run with --no-cache. As best I can tell, there's no need to disregard the cache by default. We don't pin other dependencies in the image, so it's not like avoiding the cache ensures determinism; it just ensures agreement with other machines that attempt to regenerate the files at roughly the same time. I'll see about making that an opt-in.

[brandjon]

Reopening for some continued cleanups...