build(docker): reduce node image size

[xendarboh]

use upx-compressed binaries and look for other image reduction opportunities

$ docker run --rm -ti zkn/node:latest /bin/bash -c 'du -hs /opt/zkn'
189M    /opt/zkn

$ docker run --rm -ti zkn/node:latest /bin/bash -c 'ls -l /opt/zkn'
total 192824
-rwxr-xr-x 1 root root  4277126 Nov 16 04:36 echo_server
-rwxr-xr-x 1 root root 20298816 Nov 16 04:35 fetch
-rwxr-xr-x 1 root root 16197912 Nov 16 04:36 genconfig
-rwxr-xr-x 1 root root  8393787 Nov 16 04:36 http_proxy
-rwxr-xr-x 1 root root 13058480 Nov 16 04:36 memspool
-rwxr-xr-x 1 root root  7234457 Nov 16 04:36 panda_server
-rwxr-xr-x 1 root root  5063476 Nov 16 04:36 pigeonhole
-rwxr-xr-x 1 root root 20338504 Nov 16 04:36 ping
-rwxr-xr-x 1 root root 20721512 Nov 16 04:36 pki
-rwxr-xr-x 1 root root 21299912 Nov 16 04:36 proxy_client
-rwxr-xr-x 1 root root  8017701 Nov 16 04:36 proxy_server
-rwxr-xr-x 1 root root 30468792 Nov 16 04:36 server
-rwxr-xr-x 1 root root 22049840 Nov 16 04:36 walletshield