Configure Minio to run as non-root user

Two changes occur here:

• Addition of securityContext to the Pod
  • This sets the running user to 1000, which is not required to exist inside the container.
  • Sets the fsGroup to 1000, which k8s will automatically ensure that the Pod's PV contents are automatically chown 1000:1000
  • The combination of the two lets us run as non-root, with file permissions automatically handled.
• Modification of the default value when minio.configPath is not provided (default from values.yaml`
  • Here, we set this to /tmp/.minio in place of /root/.minio. This relies on the standard behaviors of the filesystems, which sets /tmp as accessible for all users.

Upside of this approach:

• minimal modifications compared to the upstream charts. Easier to maintain the fork.
• No need for alterations to the container image in use. Less items to maintain overall.

Closes #268 (closed)