docker: fixes ui example which extracted lens to the wrong path

[codefromthecrypt]

We introduced a glitch I'm not sure when which extracted the UI to the wrong directory in our zipkin-ui test image

This would 404 because the index isn't at /index.html rather /zipkin-lens/index.html

This fixes that, as well some glitches I noticed in the build, for example being no longer able to build this in alpine.

Before

$ docker run --rm --entrypoint /bin/sh  ghcr.io/openzipkin/zipkin-ui:3.1.0 -c 'find /var/www/html'
/var/www/html
/var/www/html/zipkin
/var/www/html/zipkin/META-INF
/var/www/html/zipkin/META-INF/MANIFEST.MF
/var/www/html/zipkin/zipkin-lens
/var/www/html/zipkin/zipkin-lens/static
/var/www/html/zipkin/zipkin-lens/static/js
/var/www/html/zipkin/zipkin-lens/static/js/index.51c4d5b0.js
/var/www/html/zipkin/zipkin-lens/static/media
/var/www/html/zipkin/zipkin-lens/static/media/zipkin-logo.76388cfe.png
/var/www/html/zipkin/zipkin-lens/static/css
/var/www/html/zipkin/zipkin-lens/static/css/index.a7aa86b4.css
/var/www/html/zipkin/zipkin-lens/favicon.ico
/var/www/html/zipkin/zipkin-lens/index.html

After

$ docker run --rm --entrypoint /bin/sh  openzipkin/zipkin-ui:test -c 'find /var/www/html'
/var/www/html
/var/www/html/zipkin
/var/www/html/zipkin/static
/var/www/html/zipkin/static/js
/var/www/html/zipkin/static/js/index.51c4d5b0.js
/var/www/html/zipkin/static/media
/var/www/html/zipkin/static/media/zipkin-logo.76388cfe.png
/var/www/html/zipkin/static/css
/var/www/html/zipkin/static/css/index.a7aa86b4.css
/var/www/html/zipkin/favicon.ico
/var/www/html/zipkin/index.html