enhancements to support postgres client-cert authentication (#47)

This PR adds a few new chart features which adds to the flexibility of the chart.

- allow extra volumes to be mounted (such as secrets): 2f862c5a48
- pass environment variables also to the init-container: 7044049478
- allow a preparation script to be "injected" into the init-container: 6125a69345

As a concrete example of how this can be used, I use is to configure Gitea to use client certificate authentication against an external Postgres database. That could be accomplished by having a `gitea-postgres-ssl` secret:

```
apiVersion: v1
kind: Secret
type: Opaque
metadata:
  name: gitea-postgres-ssl
data:
  postgresql.crt: <base64...>
  postgresql.key: <base64...>
  root.crt: <base64...>
```

and then mounting this as a volume in Gitea using:

```
extraVolumes:
- name: postgres-ssl-vol
  secret:
    secretName: gitea-postgres-ssl

extraVolumeMounts:
- name: postgres-ssl-vol
  readOnly: true
  mountPath: "/pg-ssl"
```

To get the right permissions on the credentials, we'd use the `initPreScript`:

```
initPreScript: |
  # copy postgres client and CA cert from mount and
  # give proper permissions
  mkdir -p /data/git/.postgresql
  cp /pg-ssl/* /data/git/.postgresql/
  chown -R git:git /data/git/.postgresql/
  chmod 400 /data/git/.postgresql/postgresql.key
```

and to make sure that Gitea uses the certificate we need to pass the proper postgres environment variables (both to the init container and the "main" container):

```
statefulset:
  env:
  - name:  "PGSSLCERT"
    value: "/data/git/.postgresql/postgresql.crt"
  - name:  "PGSSLKEY"
    value: "/data/git/.postgresql/postgresql.key"
  - name:  "PGSSLROOTCERT"
    value: "/data/git/.postgresql/root.crt"
```

Co-authored-by: Peter Gardfjäll <peter.gardfjall.work@gmail.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/47
Reviewed-by: luhahn <luhahn@noreply.gitea.io>
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: petergardfjall <petergardfjall@noreply.gitea.io>
Co-committed-by: petergardfjall <petergardfjall@noreply.gitea.io>

values.yaml

@@ -69,11 +69,36 @@ statefulset:
 
 persistence:
   enabled: true
-  # existingClaim: 
+  # existingClaim:
   size: 10Gi
   accessModes:
     - ReadWriteOnce
 
+# additional volumes to add to the Gitea statefulset.
+extraVolumes:
+# - name: postgres-ssl-vol
+#   secret:
+#     secretName: gitea-postgres-ssl
+
+
+# additional volumes to mount, both to the init container and to the main
+# container. As an example, can be used to mount a client cert when connecting
+# to an external Postgres server.
+extraVolumeMounts:
+# - name: postgres-ssl-vol
+#   readOnly: true
+#   mountPath: "/pg-ssl"
+
+# bash shell script copied verbatim to the start of the init-container.
+initPreScript: ""
+#
+# initPreScript: |
+#   mkdir -p /data/git/.postgresql
+#   cp /pg-ssl/* /data/git/.postgresql/
+#   chown -R git:git /data/git/.postgresql/
+#   chmod 400 /data/git/.postgresql/postgresql.key
+
+
 gitea:
   admin:
     username: gitea_admin
@@ -96,8 +121,8 @@ gitea:
 
   config: {}
   #  APP_NAME: "Gitea: Git with a cup of tea"
-  #  RUN_MODE: dev   
-  #   
+  #  RUN_MODE: dev
+  #
   #  server:
   #    SSH_PORT: 22
   #