I’ve been getting my custom bits working with the latest HASS including running in a docker environment. That includes making changes to my custom panel. The changes never showed up in the browser. All caching was disabled. I could even start a shell in the container and see that the file was showing up as different there but when HASS sent the file, it was an old version. Even restarting the container didn’t change anything. That was the opposite of what containers are good for.
After some time wondering if I was simply nuts, I dug down into aiohttp and found a call to os.sendfile that was spitting out old (and non-present) data. Google found me https://www.virtualbox.org/ticket/9069.
The fix is to add AIOHTTP_NOSENDFILE=1 to your container environment.
Note that this will only affect Docker Toolbox users who have docker-machine spinning up a VirtualBox VM on your host.