Create Kubernetes deployment and service from image using configmap and secret - postgresql

Question

I am trying to create a deployment from an image that I specify in a Dockerfile. My goal would be to pass the environmental variables so I could access them inside the Dockerfile, so kind of similar to the postgres image available on Dockerhub. Additionally I would need a service so I could access the database in a Spring boot application. The reason why I would like to work it this way, because I would like to have additional content inside the Dockerfile. I am not sure whether I can do it this way, but if yes, what am I doing wrong, and how could I access it from Spring boot?
What I have tried so far:
apiVersion: v1
kind: Namespace
metadata:
name: testns
---
apiVersion: v1
kind: ConfigMap
metadata:
name: postgres-configmap
namespace: testns
data:
dbname: test_database
---
apiVersion: v1
kind: Secret
metadata:
name: postgres-secret
namespace: testns
type: Opaque
data:
username: cG9zdGdyZXN1c2Vy
password: cGFzc3dvcmQxMjM0NTY=
---
apiVersion: apps/v1
kind: Deployment
metadata:
name: postgres-depl
namespace: testns
labels:
app: postgresql
spec:
replicas: 1
selector:
matchLabels:
app: postgresql
template:
metadata:
labels:
app: postgresql
spec:
containers:
- name: postgresdb
image: testpostgres
ports:
- containerPort: 5432
env:
- name: DB
valueFrom:
configMapKeyRef:
name: postgres-configmap
key: dbname
- name: USERNAME
valueFrom:
secretKeyRef:
name: postgres-secret
key: username
- name: PASSWORD
valueFrom:
secretKeyRef:
name: postgres-secret
key: password
---
apiVersion: v1
kind: Service
metadata:
name: postgres-service
namespace: testns
spec:
selector:
app: postgresql
type: LoadBalancer
ports:
- protocol: TCP
port: 7000
targetPort: 5432
nodePort: 30000
And the related Dockerfile that I use to create the testpostgres image:
FROM postgres
ARG DB
ARG USERNAME
ARG PASSWORD
ENV POSTGRES_DB=$DB
ENV POSTGRES_USER=$USERNAME
ENV POSTGRES_PASSWORD=$PASSWORD
EXPOSE 5432