I run nearly all my Docker workloads with their data just in the home directory of the VM (or LXC actually since that’s how I roll) I’m running them in, but a few have data on my separate NAS via and NFS share - so through a switch etc with no problems - just slowish.

Great. There’s two volumes there - firefly_iii_upload & firefly_iii_db.

You’ll definitely want to docker compose down first (to ensure the database is not being updated), then:

docker run --rm \
  -v firefly_iii_db:/from \
  -v $(pwd):/to \
  alpine sh -c "cd /from && tar cf /to/firefly_iii_db.tar ."

and

docker run --rm \
  -v firefly_iii_upload:/from \
  -v $(pwd):/to \
  alpine sh -c "cd /from && tar cf /to/firefly_iii_upload.tar ."

Then copy those two .tar files to the new VM. Then create the new empty volumes with:

docker volume create firefly_iii_db
docker volume create firefly_iii_upload

And untar your data into the volumes:

docker run --rm \
  -v firefly_iii_db:/to \
  -v $(pwd):/from \
  alpine sh -c "cd /to && tar xf /from/firefly_iii_db.tar"

docker run --rm \
  -v firefly_iii_upload:/to \
  -v $(pwd):/from \
  alpine sh -c "cd /to && tar xf /from/firefly_iii_upload.tar"

Then make sure you’ve manually brought over the compose file and those two .env files, and you should be able to docker compose up and be in business again. Good choice with Proxmox in my opinion.

I’m not clear from your question, but I’m guessing you’re talking about data stored in Docker volumes? (if they are bind mounts you’re all good - you can just copy it). The compose files I found online for FireflyIII use volumes, but Hammond looked like bind mounts. If you’re not sure, post your compose files here with the secrets redacted.

To move data out of a Docker volume, a common way is to mount the volume into a temporary container to copy it out. Something like:

docker run --rm \
  -v myvolume:/from \
  -v $(pwd):/to \
  alpine sh -c "cd /from && tar cf /to/myvolume.tar ."

Then on the machine you’re moving to, create the new empty Docker volume and do the temporary copy back in:

docker volume create myvolume
docker run --rm \
  -v myvolume:/to \
  -v $(pwd):/from \
  alpine sh -c "cd /to && tar xf /from/myvolume.tar"

Or, even better, just untar it into a data directory under your compose file and bind mount it so you don’t have this problem in future. Perhaps there’s some reason why Docker volumes are good, but I’m not sure what it is.