fix(data): Use /data for persisting files and file caches (#295)

Moves sentry-data volume to /data mount point and sets all file-based storage settings to /data/files, /data/dsym-cache etc. accordingly. See https://github.com/getsentry/sentry/blob/50ac5506669043dd939dd0e44fe4cfb00377ff1d/src/sentry/options/defaults.py#L45-L54