I have a simple Scala app which using dockerfile and docker-compose.
Dockerfile looks like:

FROM hseeberger/scala-sbt:8u222_1.3.5_2.13.1
WORKDIR /my-app
COPY . /my-app
RUN sbt compile
EXPOSE 8000 8080
CMD sbt run

docker-compose.yaml:

version: "3.3"
services:
  app:
    build: .
    ports:
      - "8080:8080"
      - "8000:8000"
  postgres:
    image: postgres:latest
    expose:
      - "5432"
    ports:
      - "5432:5432"
    hostname: postgres
    environment:
      - POSTGRES_DB=customers
      - POSTGRES_USER=postgres
      - POSTGRES_PASSWORD=postgres

build.sbt file:

import sbt.Keys.mainClass

ThisBuild / version := "0.1.0-SNAPSHOT"

ThisBuild / scalaVersion := "2.13.8"

mainClass := Some("my.app.Main")

lazy val root = (project in file("."))
  .settings(
    name := "my-app"
  )

When I ran it via docker-compose up command, I got an error:

app_1       | [info] welcome to sbt 1.6.2 (Oracle Corporation Java 1.8.0_222)
app_1       | [info] loading project definition from /opt/docker/project
app_1       | [info] loading settings for project root from build.sbt ...
app_1       | [info] set current project to my-app (in build file:/opt/docker/)
app_1       | [error] java.lang.RuntimeException: No main class detected.
app_1       | [error]   at scala.sys.package$.error(package.scala:30)
app_1       | [error] stack trace is suppressed; run 'last Compile / bgRun' for the full output
app_1       | [error] (Compile / bgRun) No main class detected.

It seems it could not find Main class even if it is defined in build.sbt file.

When I ran it just with sbt run everything works fine and Main is found correctly. The problem exists only when I want to run app with docker-compose.
Do you know how I can fix it?