Add prune and du subcommands

[ktock]

$ buildg.sh du
WARN[2022-06-01T03:08:38Z] using host network as the default            
ID									RECLAIMABLE	SIZE
mkc2l6guvrgl5f193q9gx778u                                              	true       	2112204 B
wa8gx348tg9h2ags1hy0rfdxe                                              	true       	1351680 B
nenp1s8rlyd3azm4qr2o0qfi6                                              	true       	1347584 B
29my9ijfl16gkj3p3ipnpymjm                                              	true       	8192 B
wg4kpbve5fae4uny85cw8eddc                                              	true       	4096 B
Total:	4823756 B
$ buildg.sh prune
WARN[2022-06-01T03:08:40Z] using host network as the default            
ID									RECLAIMABLE	SIZE
wa8gx348tg9h2ags1hy0rfdxe                                              	true       	1351680 B
29my9ijfl16gkj3p3ipnpymjm                                              	true       	8192 B
wg4kpbve5fae4uny85cw8eddc                                              	true       	4096 B
nenp1s8rlyd3azm4qr2o0qfi6                                              	true       	1347584 B
mkc2l6guvrgl5f193q9gx778u                                              	true       	2112204 B
Total:	4823756 B
$ buildg.sh du
WARN[2022-06-01T03:08:42Z] using host network as the default            
ID	RECLAIMABLE	SIZE
Total:	0 B