by 
I had many unused images appeared because of my fault:
Deryas-MacBook-Pro:etugra dsezen$ docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
etugra latest 0e0485e9abbf 5 minutes ago 704.1 MB
<none> <none> 8a2ffaa937f3 8 minutes ago 704.1 MB
<none> <none> d13dd077234c 9 minutes ago 704.1 MB
<none> <none> 017cb8b846b7 17 minutes ago 704.1 MB
<none> <none> f671aaefbeb8 18 minutes ago 704.1 MB
<none> <none> 280ea3ac4ae5 44 minutes ago 704.1 MB
<none> <none> 97dcf929d414 45 minutes ago 704.1 MB
<none> <none> 019bcd4d779e 53 minutes ago 704.1 MB
<none> <none> 053a969a5a22 55 minutes ago 704.1 MB
<none> <none> 94be3db991db 58 minutes ago 704.1 MB
<none> <none> cd284b979277 59 minutes ago 704.1 MB
<none> <none> 56181d2f6edf 59 minutes ago 704.1 MB
<none> <none> a55ec92c3137 About an hour ago 704.1 MB
<none> <none> 7e9afad720aa About an hour ago 704.1 MB
<none> <none> 907bbba168bf About an hour ago 704.1 MB
<none> <none> f42d87e744f6 About an hour ago 704.1 MB
<none> <none> 804aa0feedf2 About an hour ago 704.1 MB
<none> <none> 1036b6db0f20 About an hour ago 704.1 MB
<none> <none> 4868a31676cb About an hour ago 704.1 MB
<none> <none> 7ade30482774 About an hour ago 704.1 MB
<none> <none> 353b07a482be About an hour ago 704.1 MB
<none> <none> d827f90112d1 About an hour ago 704.1 MB
<none> <none> efb1e94f2c4f About an hour ago 704.1 MB
<none> <none> 9e1210fd7d8f 2 hours ago 684.3 MB
<none> <none> 63eaa30ce706 2 hours ago 684.3 MB
<none> <none> b89d5acf03e5 2 hours ago 684.3 MB
<none> <none> c9905abf50b8 2 hours ago 709.6 MB
<none> <none> af6393bc4b96 2 hours ago 696.2 MB
<none> <none> 4bb0534b3bd0 21 hours ago 675.1 MB
python 3-onbuild 61bd1f884054 11 days ago 684.3 MB
Seems docker rmi does not accept standard input, so here is the way i found so that you can overcome this:
docker images|grep none|awk {'print $3'} > /tmp/dockertmp
for i in `cat /tmp/dockertmp`; do docker rmi $i; done
