ASPNETCORE_URLS not being applied (deploying in docker container)

Question

I'm trying to have my asp.net core app use ASPNETCORE_URLS to set the launch URL. It is not working as expected.

I have tried everything I found online, but I keep getting stuck. The app works without the environmental variables and runs in a docker container fine. But wont work when enabling environmental variables.

Desired result: 0.0.0.0:5000 Result : localhost:5000

Startup:

        public Startup(IConfiguration configuration, IWebHostEnvironment env)
    {
        Configuration = new ConfigurationBuilder()
             .SetBasePath(env.ContentRootPath)
          .AddJsonFile("appsettings.json")
          .AddJsonFile($"appsettings.{env.EnvironmentName}.json")
          .AddEnvironmentVariables()
          .Build();
    }

Env variable in dockerfile:

ENV ASPNETCORE_URLS=http://+:5000

Docker File:

#See https://aka.ms/containerfastmode to understand how Visual Studio uses 

this Dockerfile to build your images for faster debugging.

FROM mcr.microsoft.com/dotnet/core/aspnet:3.1-buster-slim AS base
WORKDIR /app
EXPOSE 5000
ENV ASPNETCORE_URLS=http://+:5000

FROM mcr.microsoft.com/dotnet/core/sdk:3.1-buster AS build
WORKDIR /src
COPY ["Platform/Platform.API/Platform.API.csproj", "Platform.API/"]
COPY ["Platform/Platform.Domain/Platform.Domain.csproj", "Platform.Domain/"]
COPY ["Platform/Platform.DataAccess/Platform.DataAccess.csproj", "Platform.DataAccess/"]
RUN dotnet restore "Platform.API/Platform.API.csproj"
COPY ./Platform .
WORKDIR "/src/Platform.API"
RUN dotnet build "Platform.API.csproj" -c Release -o /app/build

FROM build AS publish
RUN dotnet publish "Platform.API.csproj" -c Release -o /app/publish

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

The environmental variable is detected by the application, it just wont be used for some reason.

Thanks in advance!

Answer 1

this is a common misconception of the ENV keyword in DOCKERFILE move it to the app image to have an effect

FROM base AS final
WORKDIR /app
ENV ASPNETCORE_URLS=http://+:5000
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "Platform.API.dll"]

the ENV keyword applies to the current build stage according to the dockerfile reference

The ENV instruction sets the environment variable to the value . This value will be in the environment for all subsequent instructions in the build stage and can be replaced inline in many as well.

FROM however starts a new build stage

The FROM instruction initializes a new build stage and sets the Base Image for subsequent instructions. As such, a valid Dockerfile must start with a FROM instruction.

you can try this by building this DOCKERFILE

from alpine
ENV asdf test
RUN echo $asdf

from alpine
RUN echo $asdf

returns

$ docker build -t envtest .
Sending build context to Docker daemon  6.656kB
Step 1/5 : from alpine
 ---> 5cb3aa00f899
Step 2/5 : ENV asdf test
 ---> Running in 91ae4904857e
Removing intermediate container 91ae4904857e
 ---> 63ef857d07a6
Step 3/5 : RUN echo $asdf       <------ works in same build stage
 ---> Running in b9037c76cc93
test
Removing intermediate container b9037c76cc93
 ---> 17edf57d8055
Step 4/5 : from alpine
 ---> 5cb3aa00f899
Step 5/5 : RUN echo $asdf       <------- does not in next build stage
 ---> Running in 62b42e7c28d8

Removing intermediate container 62b42e7c28d8
 ---> 7e6a8a58442f
Successfully built 7e6a8a58442f
Successfully tagged envtest:latest