Why is the port not forwarded in Docker?

Question

My DockerFile:

FROM mcr.microsoft.com/dotnet/aspnet:8.0 AS base
#USER app
WORKDIR /app
EXPOSE 80/tcp


FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
ARG BUILD_CONFIGURATION=Release
WORKDIR /src
COPY ["Services/Catalog/Catalog.API/Catalog.API.csproj", "Services/Catalog/Catalog.API/"]
RUN dotnet restore "./Services/Catalog/Catalog.API/./Catalog.API.csproj"
COPY . .
WORKDIR "/src/Services/Catalog/Catalog.API"
RUN dotnet build "./Catalog.API.csproj" -c $BUILD_CONFIGURATION -o /app/build

FROM build AS publish
ARG BUILD_CONFIGURATION=Release
RUN dotnet publish "./Catalog.API.csproj" -c $BUILD_CONFIGURATION -o /app/publish /p:UseAppHost=false

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "Catalog.API.dll"]

my docker-compose file

version: '3.4'

services:

  catalogdb:
    container_name: cataloggdb
    restart: always
    ports:
      - "27017:27017"
    volumes:
      - mongo_data:/data/db 

  catalog.api:
    container_name: catalog.api
    environment:
      - ASPNETCORE_ENVIRONMENT=Development
      - "DatabaseSettings:ConnectionString=mongodb://catalogdb:27017"
    depends_on:
      - catalogdb
    ports:
      - "8000:80"

I exec

docker-compose -f .\docker-compose.yml -f .\docker-compose.override.yml up -d

✔ Network src_default    Created                                                                                  0.0s
 ✔ Container cataloggdb   Started                                                                                  0.1s
 ✔ Container catalog.api  Started

When I go to http://localhost:27017/, it works:

When I go to http://localhost:8000/swagger/index.html, it doesn't work:

Answer 1

EXPOSE does not actually set port, it is basically documentation:

The EXPOSE instruction doesn't actually publish the port. It functions as a type of documentation between the person who builds the image and the person who runs the container, about which ports are intended to be published.

Since .NET 8 in ASP.NET Core images default port is 8080 - see Default ASP.NET Core port changed from 80 to 8080.

Either change the mapping to reflect that:

ports:
  - "8000:8080"

Or set port to 80 for example by using ASPNETCORE_HTTP_PORTS environment variable:

environment:
   - ASPNETCORE_HTTP_PORTS=80

Or

FROM mcr.microsoft.com/dotnet/aspnet:8.0 AS base
#USER app
WORKDIR /app
ENV ASPNETCORE_HTTP_PORTS=80
EXPOSE 80/tcp