* support cross-compiling an x86-64 target on an arm64 host. This
it turns out is a matter of *removing* an unnecessary dependency.
(aarch64-linux-gnu-pkg-config exists but x86_64-linux-gnu-pkg-config
doesn't. Turns out neither is necessary.) Added a comment explaining
where ${gcc_target}-pkg-config comes from now.
* documentation tweaks
* improve debug output a bit
* run node 12, 14, and 16 (next to be supported) on CI. This will catch
node version-specific problems like that solved in dad9bdc.
* mention 12 and 14 in build instructions and link to instructions for
installing that version.
* follow this in Dockerfile, installing version 14. This addresses
a "Cannot find module 'worker_threads'" error introduced in
39a63e0, which (inadvisedly) upgraded gzipper 4->5 in addition to
the material-ui upgrade.
* use utf-8 encoding rather than ascii in live part parser. Those
builds apparently don't support ascii. iThey must use "small-icu" or
have ICU disabled, as described here:
https://nodejs.org/api/util.html#util_encodings_supported_when_node_js_is_built_with_the_small_icu_option
Most importantly, in build-ui.bash, fix an extra "&&". This meant
that if the build command fails, it would proceed and cause confusion.
This happened for me: I ran it without the emulation installed. Both
the build-server and deploy stages had problems, but because of the "&&"
the deploy target didn't actually return failure. After I fixed the
emulation problem, there was a bad cached layer.
Also save the build output from the dev stage.
