Permission denied on Kubernetes InitContainer while running command

Question

I'm trying to add a certificate bundle to an existing certificate bundle file in a container. But I'm getting the error permission denied. I'm aware this is due to the PSP my company has initialised. It's not possible to run this container as root. Is there another way to add something to an existing file bypassing the permission issue?

This is a part of my values.yml file for the Nexus Helm chart:

  initContainers:
    - name: ca-store
      image: ******/docker.io/library/busybox:1.34.1
      volumeMounts:
        - name: ca-store
          mountPath: /usr/local/share/ca-certificates/tls-ca-bundle.crt
          subPath: tls-ca-bundle.crt
          readOnly: true
      # command: ['/bin/sh', '-c', 'cd /etc/ssl/certs && ls -lhrt']
      command: ['/bin/sh', '-c', 'cat /usr/local/share/ca-certificates/tls-ca-bundle.crt >> /etc/ssl/certs/ca-certificates.crt']
  postStart:
    command: # '["/bin/sh", "-c", "cat /usr/local/share/ca-certificates/tls-ca-bundle.crt >> /etc/ssl/certs/ca-certificates.crt"]'
  preStart:
    command: # '["/bin/rm", "-f", "/path/to/lockfile"]'

  additionalVolumes:
    - name: ca-store
      configMap:
        name: ca-store
  additionalVolumeMounts:
    # - name: ca-store
    #   mountPath: /usr/local/share/ca-certificates/tls-ca-bundle.crt
    #   subPath: tls-ca-bundle.crt
    #   readOnly: true

So anybody ideas?