feat: Add ability to inject extraVolumeMounts into UI container

[mkilchhofer]

Follow-up of discussion in PR:

• feat: Add ability to define securityContexts and podSecurityContext #38

Just as a note to anyone leveraging this later, the UI container crashes if readOnlyRootFilesystem is true as it needs to write to tmp space.

[emerg] 1#1: mkdir() "/tmp/proxy_temp" failed (30: Read-only file system)

Source: @toscott , #38 (comment)

[toscott]

Looks good! Was able to test it out with a read only root filesystem on the ui container and everything starts up. Also ran it with no values and everything starts up clean.

Example configuration with UI read only file system.

podSecurityContext:
  fsGroup: 2000

opencost:
  ui:
    extraVolumeMounts:
      - mountPath: /tmp
        name: test-volume
    securityContext:
      capabilities:
        drop:
        - ALL
      readOnlyRootFilesystem: true
      runAsNonRoot: true
      runAsUser: 1000
extraVolumes:
  - name: test-volume
    emptyDir:
      sizeLimit: 10Mi