dockerfile tweaks

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.

docker/build-ui.bash

@@ -12,7 +12,11 @@ set -o xtrace
 mkdir /docker-build-debug/build-ui
 exec > >(tee -i /docker-build-debug/build-ui/output) 2>&1
 
+date
+uname -a
 time npm ci
 time npm run build
+
 ls -laFR /var/lib/moonfire-nvr/src/ui/node_modules \
     > /docker-build-debug/build-ui/node_modules-after
+date