How to understand what is inside unnamed docker images?

Question

Docker

[email protected]:~# docker images --all                                                                                                                      
REPOSITORY                    TAG                 IMAGE ID            CREATED             SIZE                                                             
lluisyast/freepbx             latest              461646ee215d        13 days ago         1.988 GB                                                         
selenium/standalone-chrome    3.0.1-carbon        9ddcca34a1ce        2 weeks ago         794 MB                                                           
<none>                        <none>              2d1eb247d496        4 weeks ago         649.4 MB                                                         
<none>                        <none>              390125ce0b22        4 weeks ago         649.4 MB                                                         
<none>                        <none>              62a7cf2001a9        4 weeks ago         649.4 MB                                                         
<none>                        <none>              28c4638b8013        4 weeks ago         649.4 MB                                                         
<none>                        <none>              722b065f63fa        4 weeks ago         649.4 MB                                                         
<none>                        <none>              f20ce7f3c8d9        4 weeks ago         649.4 MB                                                         
<none>                        <none>              4edb99066226        4 weeks ago         649.3 MB                                                         
<none>                        <none>              8902e11f48be        4 weeks ago         649.3 MB                                                         
<none>                        <none>              9256a7e0eb75        4 weeks ago         649.3 MB                                                         
<none>                        <none>              efceb2db5ce3        4 weeks ago         649.3 MB                                                         
<none>                        <none>              7ba93665a16b        4 weeks ago         649.3 MB                                                         
<none>                        <none>              8fc700d6a782        4 weeks ago         649.3 MB                                                         
<none>                        <none>              680d555f19bb        4 weeks ago         649.3 MB                                                         
<none>                        <none>              6bb5d1812c55        4 weeks ago         645.9 MB                                                         
<none>                        <none>              f8b7828174d9        4 weeks ago         645.9 MB                                                         
<none>                        <none>              4ba2a7007dd8        4 weeks ago         645.9 MB                                                         
node                          argon               e834398209c1        5 weeks ago         645.9 MB                                                         
node                          latest              89cd8193c9a1        5 weeks ago         655.5 MB                                                         
selenium/standalone-firefox   2.53.0              0067b27644ee        3 months ago        598.4 MB                                                         
voipnovatos/asterisk-13       latest              ae1b3a5713a8        5 months ago        935.2 MB                                                         
[email protected]:~# df -h                                                                                                                                    
Filesystem      Size  Used Avail Use% Mounted on                                                                                                           
udev            237M     0  237M   0% /dev                                                                                                                 
tmpfs            49M  3.1M   46M   7% /run                                                                                                                 
/dev/vda1        20G   11G  8.3G  56% /                                                                                                                    
tmpfs           245M     0  245M   0% /dev/shm                                                                                                             
tmpfs           5.0M     0  5.0M   0% /run/lock                                                                                                            
tmpfs           245M     0  245M   0% /sys/fs/cgroup                                                                                                       
/dev/dm-2        10G  1.6G  8.5G  16% /var/lib/docker/devicemapper/mnt/fecfdae4bfbd7295069f718d613f9228e0afc8dd61614fa0ebeb16b11508cb00                    
shm              64M     0   64M   0% /var/lib/docker/containers/6c7743eca5c0984ee789f8fad4337b75a16c9dbf1a076f6490334c9db581763f/shm                      
tmpfs            49M     0   49M   0% /run/user/0

Answer 1

These are intermediate dependencies for tagged images. They are automatically removed when the last tagged image that depends on the given layer is removed.
And if you just need to look at the insides, then the easiest way is to create a container from the image of interest.
Or take advantagedocker history