add the filebrowser rock and tests

[ca-scribner]

This PR adds:

• filebrowser/rockcraft.yaml, which implements a rock for filebrowser
• filebrowser/README.md with some notes and instructions for future maintenance
• filebrowser/tox.ini and related sanity tests

Closes canonical/kubeflow-rocks#72

Testing instructions:

testing the rock itself:

cd filebrowser
tox -e pack
tox -e export-to-docker
docker run -p 8080:80 -e FB_ADDRESS=0.0.0.0 filebrowser:v2.27.0
# Browse in a browser to `localhost:8080` and see the app

(if you want to) testing in charmed kubeflow

# with a fully deployed charmed kubeflow
juju config kubeflow-volumes volume-viewer-image=local/filebrowser:v2.27.0

tox -e export-to-docker
docker tag filebrowser:v2.27.0 local/filebrowser:v2.27.0  # so we really know its the right image
docker save local/filebrowser:v2.27.0 > local_filebrowser_v2.27.0.tar
microk8s ctr image import local_filebrowser_v2.27.0.tar

then

• log into 10.64.140.43.nip.io using user123/user123
• go to volumes web app
• create a volume named testvol
• click the "Open PVC Viewer" button (small folder icon on the right side of the row)
• filebrowser window should open (although there will be no files since pvc is empty - you can create some though!)
• confirm we got a filebrowser with the new image:

kubectl -n user123 get pod $(kubectl get pod -n user123 --selector=app.kubernetes.io/instance=pvcviewer-testvol --output=jsonpath={.items..metadata.name}) -o yaml | grep filebrowser:
# Should return:
filebrowser:
    image: docker.io/library/local/filebrowser:v2.27.0
    image: docker.io/library/local/filebrowser:v2.27.0

# NOT
filebrowser:
    image: docker.io/library/filebrowser/filebrowser:v2.23.0
    image: docker.io/library/filebrowser/filebrowser:v2.23.0

[ca-scribner]

Hit a lot of issues while creating this rock. They're documented in the rock and included README.md, but in summary the rock:

• uses a few atypical build procedures (GoReleaser, and also its go build embeds the frontend into the application)
• a healthcheck script and HEALTHCHECK directive in the dockerfile
• needs somewhere with write access for a local db
• needs a config file to define defaults to both the healthcheck and filebrowser binary
• Charmed Kubeflow uses it via a controller we don't control, so we need to emulate the API of this image to make it a drop-in replacement of the upstream docker image. This means putting files in consistent places and setting a default entrypoint-service

Along the way I also got stuck with rockcraft in a few places:

• misunderstanding how to pass args for the entrypoint-service
• finding out that rocks don't automatically stage .dotfiles
• finding a pattern for passing build artifacts between parts
• finding that stage clobbers stage-packages, meaning that if you set stage you no longer get your stage-packages
• figuring out how permissions work
• clarifying how different directories work in rockcraft

[orfeas-k]

Great job @ca-scribner . This had quite a few hard spots that needed to go around. I left some comments

[orfeas-k]

Thank you Andrew, very good job!

[ca-scribner]

it has been raised in canonical/craft-parts#427