fix(server): mounted fs path for static-ui-dir

[parkedwards]

resolves #288

$ kl prefect-server-5dd4b5755-7b5k7

 ___ ___ ___ ___ ___ ___ _____
| _ \ _ \ __| __| __/ __|_   _|
|  _/   / _|| _|| _| (__  | |
|_| |_|_\___|_| |___\___| |_|

Configure Prefect to communicate with the server with:

    prefect config set PREFECT_API_URL=http://0.0.0.0:4200/api

View the API reference documentation at http://0.0.0.0:4200/docs

Check out the dashboard at http://0.0.0.0:4200

[parkedwards]

this needs a default value, or else the server container will look for a non-existent default that is baked into the application code. so the expectation is, when using the helm chart, we will be setting this by either user-provided value or this default value

[jimid27]

nice

[parkedwards]

nothing really special about this directory, so we can re-use the emptyDir that we initialize here