CRITICAL BUG: DDev always caches VRT reference images, so opening a report after fresh reference snapshot results in strange behaviour

[dalin-]

Steps to reproduce

1. Run ddev backstop reference to get some "reference" snapshots (e.g. of the live site)
2. Open the backstop browser report.
3. Do something to change the content in the reference (either make a content change on the live site, or set a different environment as the reference)
4. Rerun ddev backstop reference to get fresh "reference" snapshots
5. Open the backstop browser report. Note how the "reference" images are from the first run and not your changed content, even though backstop is diffing correctly. Note how the "test" images are fine, that's because they always have a new path .../bitmaps_test/<date+time>/<scenario>.png, but "reference" images always have a constant path .../bitmaps_reference/<scenario>.png.

The problem that ddev is telling the browser to cache the images:

Can we tell DDev not to cache these paths?

[dalin-]

I also report upstream, but that'll take a while to get fixed:
garris/BackstopJS#1600