clarify support for node 12 and 14

* 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

docker/build-ui.bash

@@ -14,6 +14,8 @@ exec > >(tee -i /docker-build-debug/build-ui/output) 2>&1
 
 date
 uname -a
+node --version
+npm --version
 time npm ci
 time npm run build