Why can't my Spring Boot app connect to PostgreSQL when running in Docker?

I'm trying to learn Spring Boot (and Java) and I'm running my application and database in docker containers. The problem is when I build my containers it crashes with these kind of exceptions:

#0 13.79 2023-05-29T17:41:51.480Z  INFO 81 --- [           main] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Starting...
#0 14.80 2023-05-29T17:41:52.481Z ERROR 81 --- [           main] com.zaxxer.hikari.pool.HikariPool        : HikariPool-1 - Exception during pool initialization.
#0 14.80
#0 14.80 org.postgresql.util.PSQLException: The connection attempt failed.

...

#0 14.80 Caused by: java.net.UnknownHostException: db

...

#0 14.80 2023-05-29T17:41:52.486Z  WARN 81 --- [           main] o.h.engine.jdbc.spi.SqlExceptionHelper   : SQL Error: 0, SQLState: 08001
#0 14.80 2023-05-29T17:41:52.486Z ERROR 81 --- [           main] o.h.engine.jdbc.spi.SqlExceptionHelper   : The connection attempt failed.
#0 14.80 2023-05-29T17:41:52.490Z ERROR 81 --- [           main] j.LocalContainerEntityManagerFactoryBean : Failed to initialize JPA EntityManagerFactory: [PersistenceUnit: default] Unable to build Hibernate SessionFactory; nested exception is org.hibernate.exception.JDBCConnectionException: Unable to open JDBC Connection for DDL execution [The connection attempt failed.] [n/a]

My docker-compose.yml looks like this:

services:
  backend:
    build: .
    ports:
      - 8080:8080
    environment:
      - POSTGRES_DB=example
    networks:
      - spring-postgres
    depends_on:
      - db
  db:
    image: postgres
    restart: always
    secrets:
      - db-password
    volumes:
      - db-data:/var/lib/postgresql/data
    networks:
      - spring-postgres
    environment:
      - POSTGRES_DB=example
      - POSTGRES_PASSWORD_FILE=/run/secrets/db-password
    expose:
      - 5432
  pgadmin:
    image: dpage/pgadmin4
    ports:
      - 5555:80
    volumes:
      - pgadmin4:/var/lib/pgadmin
    environment:
      - [email protected]
      - PGADMIN_DEFAULT_PASSWORD=admin
    depends_on:
      - db
    networks:
      - spring-postgres
    restart: unless-stopped
  
volumes:
  db-data:
  pgadmin4:
secrets:
  db-password:
    file: db/password.txt
networks:
  spring-postgres:

and my application.properties:

spring.jpa.properties.hibernate.dialect = org.hibernate.dialect.PostgreSQLDialect
spring.jpa.hibernate.ddl-auto=create
spring.jpa.hibernate.show-sql=true

spring.datasource.url=jdbc:postgresql://db:5432/${POSTGRES_DB}
spring.datasource.username=postgres
spring.datasource.password=${POSTGRES_PASSWORD:db-wrz2z}
spring.datasource.initialization-mode=always
spring.datasource.initialize=true
spring.datasource.continue-on-error=true

Here's also my pom.xml if it helps:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.1.0</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.nyrkes</groupId>
    <artifactId>xxx</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>xxx</name>
    <description>xxx</description>
    <properties>
        <java.version>17</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-security</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-devtools</artifactId>
            <scope>runtime</scope>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.postgresql</groupId>
            <artifactId>postgresql</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.security</groupId>
            <artifactId>spring-security-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <configuration>
                    <excludes>
                        <exclude>
                            <groupId>org.projectlombok</groupId>
                            <artifactId>lombok</artifactId>
                        </exclude>
                    </excludes>
                    <excludeDevtools>false</excludeDevtools>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

If I change the spring.jpa.hibernate.ddl-auto=create to none the build is successfull.