You have to mount the SQL file as a volumen from a configmap and use the psql cli to execute the commands from mounted file.

To execute commands from file you can change the command parameter on the yaml by this:

psql -a -f sqlCommand.sql

The configmap needs to be created using the file you pretend to mount more info here

kubectl create configmap sqlCommands.sql --from-file=sqlCommands.sql

Then you have to add the configmap and the mount statement on your job yaml and modify the command to use the mounted file.

apiVersion: batch/v1
kind: Job
metadata:
  name: init-db
spec:
  template:
    metadata:
      name:  init-db
      labels:
        app: init-postgresdb
    spec:
      containers:
      - image: "docker.io/bitnami/postgresql:11.5.0-debian-9-r60"
        name: init-db
        command: [ "bin/sh", "-c", "psql -a -f /sqlCommand.sql" ]
        volumeMounts:
        - name: sqlCommand
          mountPath: /sqlCommand.sql
        env:
          - name: DB_HOST
            value: "knotted-iguana-postgresql"
          - name: DB_DATABASE
            value: "postgres"
      volumes:
        - name: sqlCommand
          configMap:
          # Provide the name of the ConfigMap containing the files you want
          # to add to the container
          name: sqlCommand.sql
      restartPolicy: OnFailure

You will have to do the following:

1. Make the content of the SQL file as a ConfigMap like this, for example:

   apiVersion: v1
   kind: ConfigMap
   metadata:
     name: mariadb-config
   data:
     mariadb-schema: "DROP DATABASE IF EXISTS test;\n\nCREATE DATABASE IF NOT EXISTS test;
   

2. Make a volume from this config map in your deployment yaml like this:

    volumes:
    - name: mariadb-schema-config-vol
      configMap:
        name: mariadb-config
        defaultMode: 420
        items:
        - key: mariadb-schema
          path: mariadb-schema.sql
   

   And volume mount like this:

    volumeMounts:
    - mountPath: /var/db/config
      name: mariadb-schema-config-vol
   

Then your init container command will be like:

    ['sh', '-c', 'psql -a -f /var/db/config/mariadb-config.sql']

For your second question, make a shell script that reads the env variables (The db credentials - I am presuming that you are having them in secrets and using them as env variables) and then invoke this command:

   psql -a -f /var/db/config/mariadb-config.sql

So to make this happen the content of this script should be in a config map and execute the script from a volume mount, just like the above example.

Hope this helps.