Cleaning up Docker Resources

By the end of this exercise, you should be able to:

Assess how much disk space docker objects are consuming
Use docker prune commands to clear out unneeded docker objects
Apply label based filters to prune commands to control what gets deleted in a cleanup operation

Find out how much memory Docker is using by executing:
```
[centos@node-3 ~]$ docker system df
```
The output will show us how much space images, containers and local volumes are occupying and how much of this space can be reclaimed.
Reclaim all reclaimable space by using the following command:
```
[centos@node-3 ~]$ docker system prune
```
Answer with y when asked if we really want to remove all unused networks, containers, images and volumes.

Create a couple of containers with labels (these will exit immediately; why?):

[centos@node-3 ~]$ docker container run --label apple --name fuji -d alpine
[centos@node-3 ~]$ docker container run --label orange --name clementine -d alpine

Delete only those stopped containers bearing the apple label:

[centos@node-3 ~]$ docker container ls -a
[centos@node-3 ~]$ docker container prune --filter 'label=apple'
[centos@node-3 ~]$ docker container ls -a

Only the container named clementine should remain after the targeted prune.

Finally, prune containers launched before a given timestamp using the until filter; start by getting the current RFC 3339 time (https://tools.ietf.org/html/rfc3339 - note Docker requires the otherwise optional T separating date and time), then creating a new container:
```
[centos@node-3 ~]$ TIMESTAMP=$(date --rfc-3339=seconds | sed 's/ /T/')
[centos@node-3 ~]$ docker container run --label tomato --name beefsteak -d alpine
```
And use the timestamp returned in a prune:
```
[centos@node-3 ~]$ docker container prune -f --filter "until=$TIMESTAMP"
[centos@node-3 ~]$ docker container ls -a
```
Note the -f flag, to suppress the confirmation step. label and until filters for pruning are also available for networks and images, while data volumes can only be selectively pruned by label; finally, images can also be pruned by the boolean dangling key, indicating if the image is untagged.

Conclusion

In this exercise, we saw some very basic docker prune usage - most of the top-level docker objects have a prune command (docker container prune, docker volume prune etc). Most docker objects leave something on disk even after being shut down; consider using these cleanup commands as part of your cluster maintenance and garbage collection plan, to avoid accidentally running out of disk on your Docker hosts.