Add deploy box cleanup instructions

[arkid15r]

Purpose/Implementation Notes

Removing 30d+ old Docker images on every deploy would make sure we have enough disk space for the next one while keeping relatively fresh images in place.

This will address

#18 ERROR: write /var/lib/buildkit/runc-overlayfs/snapshots/snapshots/4383/fs/usr/local/lib/R/site-library/pd.genomewidesnp.5/extdata/pd.genomewidesnp.5.sqlite: no space left on device

type of issues.

Types of changes

What types of changes does your code introduce?

• Bugfix (non-breaking change which fixes an issue)

Functional tests

List out the functional tests you've completed to verify your changes work locally.

Checklist

Put an x in the boxes that apply.

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

[davidsmejia]

LGTM