You have multiple way:

The official postgresql docker image state (the 'Initialization scripts' section):

If you would like to do additional initialization in an image derived from this one, add one or more .sql, .sql.gz, or *.sh scripts under /docker-entrypoint-initdb.d

Those scripts are only run if the database is created. eg if you start/restart a pod with a data volume containing an already existing database, those scripts will not be launched.

With kubernetes, you can provide a configmap with the needed file (if file sizes below 1Mb) or provide a volume with your initialization file.

An another option can be the application itself. For instance you may use flywayDB or liquibase embeded in your application (springboot do that transparently).

this may help (here I have added configmap, persistent volume, persistent volume-claim, and Postgres deployment yaml. This yaml will automatically create a table named users in the Postgres database inside the Postgres-container. Thanks

apiVersion: v1
kind: ConfigMap
metadata:
  name: postgres-config
  labels:
    app: postgres
data:
  Postgres_DB: postgresdb

---
apiVersion: v1
kind: Secret
metadata:
  name: postgres-secret
data:
  Postgres_User: postgresadmin
  Postgres_Password: admin123

---
kind: PersistentVolume
apiVersion: v1
metadata:
  name: postgres-pv-volume
  labels:
    type: local
    app: postgres
spec:
  storageClassName: manual
  capacity:
    storage: 5Gi
  accessModes:
    - ReadWriteMany
  hostPath:
    path: "/mnt/data"

---
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: postgres-pv-claim
  labels:
    app: postgres
spec:
  storageClassName: manual
  accessModes:
    - ReadWriteMany
  resources:
    requests:
      storage: 5Gi

---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: postgres
spec:
  replicas: 1
  selector:
    matchLabels:
      app: postgres
  template:
    metadata:
      labels:
        app: postgres
    spec:
      containers:
        - name: postgres-container
          image: postgres:latest
          imagePullPolicy: "IfNotPresent"
          lifecycle:
            postStart:
              exec:
                command: ["/bin/sh","-c","sleep 20 && PGPASSWORD=$POSTGRES_PASSWORD psql -w -d $POSTGRES_DB -U $POSTGRES_USER -c 'CREATE TABLE IF NOT EXISTS users (userid SERIAL PRIMARY KEY,username TEXT,password TEXT,token TEXT,type TEXT);'"]
          ports:
            - containerPort: 5432
          env:
            - name: POSTGRES_DB
              valueFrom:
                configMapKeyRef:
                  name: postgres-config
                  key: Postgres_DB
            - name: POSTGRES_USER
              valueFrom:
                secretKeyRef:
                  name: postgres-secret
                  key: Postgres_User
            - name: POSTGRES_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: postgres-secret
                  key: Postgres_Password               
          volumeMounts:
            - mountPath: /var/lib/postgresql/data
              name: postgredb
      volumes:
        - name: postgredb
          persistentVolumeClaim:
            claimName: postgres-pv-claim