feat(helm): add value to customise PostgreSQL docker image (reanahub#774)

Closes reanahub#773

Author: mdonadoni

helm/reana/README.md

@@ -9,6 +9,7 @@ This Helm automatically prefixes all names using the release name to avoid colli
 | Parameter                                                | Description                                                                          | Default value                                   |
 |----------------------------------------------------------|--------------------------------------------------------------------------------------|-------------------------------------------------|
 | `components.reana_db.enabled`                            | Instantiate a PostgreSQL database inside the cluster                                 | true                                            |
+| `components.reana_db.image`                              | [PostgreSQL image](https://hub.docker.com/_/postgres) to use                         | `docker.io/library/postgres:12.13`              |
 | `components.reana_job_controller.image`                  | [REANA-Job-Controller image](https://hub.docker.com/r/reanahub/reana-job-controller) to use  | `docker.io/reanahub/reana-job-controller:<chart-release-version>` |
 | `components.reana_message_broker.image`                  | [REANA-Message-Broker image](https://hub.docker.com/r/reanahub/reana-message-broker) to use | `docker.io/reanahub/reana-message-broker:<chart-release-version>` |
 | `components.reana_message_broker.imagePullPolicy`        | REANA-Message-Broker image pull policy                                               | IfNotPresent                                    |

helm/reana/templates/reana-db.yaml

@@ -31,7 +31,7 @@ spec:
     spec:
       containers:
       - name: db
-        image: docker.io/library/postgres:12.13
+        image: {{ .Values.components.reana_db.image }}
         args:
         - -c
         - max_connections=300

helm/reana/values.yaml

@@ -79,6 +79,7 @@ components:
     imagePullPolicy: IfNotPresent
     image: docker.io/reanahub/reana-ui:0.9.3
   reana_db:
+    image: docker.io/library/postgres:12.13
     enabled: true
   reana_server:
     imagePullPolicy: IfNotPresent