How to get the resolved network of kafka inside docker?

[Anto Aravinth <an...@gmail.com>]

I have a spring application, which should connect to the kafka. This is
what my Docker file looks for spring application:

```
FROM maven:3-jdk-11 as builder
# create app folder for sources
RUN mkdir -p /build
WORKDIR /build
COPY pom.xml /build
#Download all required dependencies into one layer
RUN mvn -B dependency:resolve dependency:resolve-plugins
#Copy source code
COPY src /build/src
# Build application
RUN mvn package

FROM openjdk:8-jdk-alpine
RUN addgroup -S spring && adduser -S spring -G spring
USER spring:spring
ARG JAR_FILE=target/*.jar
COPY ${JAR_FILE} app.jar
ENTRYPOINT ["java","-jar","/app.jar"]
```

The kafka/Postgres/zookeeper everything other, comes from Docker images. So
thought will run the application in docker compose, so that it looks like
the following:

    version: '3.1'
    services:
        postgres:
            image: debezium/postgres
            environment:
              POSTGRES_PASSWORD: qwerty
              POSTGRES_USER: appuser
            volumes:
               - ./postgres:/data/postgres
            ports:
              - 6532:6532
        zookeeper:
            image: confluentinc/cp-zookeeper
            ports:
              - "2181:2181"
            environment:
              ZOOKEEPER_CLIENT_PORT: 2181
        kafka:
            image: confluentinc/cp-kafka
            depends_on:
              - zookeeper
              - postgres
            ports:
              - "9092:9092"
            environment:
              KAFKA_ZOOKEEPER_CONNECT: zookeeper:2181
              KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://kafka:9092
              KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR: 1
              KAFKA_LOG_CLEANER_DELETE_RETENTION_MS: 5000
              KAFKA_BROKER_ID: 1
              KAFKA_MIN_INSYNC_REPLICAS: 1
              KAFKA_ADVERTISED_HOST_NAME: kafka
        connector:
            image: debezium/connect:latest
            ports:
              - "8083:8083"
            environment:
              GROUP_ID: 1
              CONFIG_STORAGE_TOPIC: my_connect_configs
              OFFSET_STORAGE_TOPIC: my_connect_offsets
              BOOTSTRAP_SERVERS: kafka:9092
            depends_on:
              - zookeeper
              - postgres
              - kafka
        app-server:
        # Configuration for building the docker image for the backend
service
            build:
              context: . # Use an image built from the specified dockerfile
in the `polling-app-server` directory.
              dockerfile: Dockerfile
            ports:
              - "8080:8080" # Forward the exposed port 8080 on the
container to port 8080 on the host machine
            restart: always
            depends_on:
              - kafka
              - zookeeper
              - postgres
              - connector
            environment:
              BOOTSTRAP_SERVERS: kafka:9092

I pass an environment variable `BOOTSTRAP_SERVERS` with value as
`kafka:9092` (since `localhost:9092` doesn't work inside my docker
environment).

And in my spring code, I get the value like the following:

```
System.getenv("bootstrap.servers")
// or
System.getenv("BOOTSTRAP_SERVERS")
```

however, when I run `docker-compose up`, I get the value as `null` for the
above Java code. Not sure, what is the best way to get the docker resolved
network for my kafka.

 So how to fix it so that java code picks up the Kafka broker inside the
docker environment?