ASPNET Core 3.1 - Dockerfile changes don´t work when run in Visual Studio

Question

I run a web api (ASPNETCore 3.1 - API) in Visual Studio 2019 (F5 in Debug mode), I use Docker (for Linux), when I change Dockerfile and run application, the changes don´t work.

Example: I put the RUN touch test.txt and ENV my_variable value in Dockerfile, but the file and variable are not created.

Does anyone know why?

Answer 1

Nothing like reading the official documentation...

https://learn.microsoft.com/en-us/visualstudio/containers/container-build?view=vs-2019#debugging

Debugging

When building in Debug configuration, there are several optimizations that Visual Studio does that help with the performance of the build process for containerized projects. The build process for containerized apps is not as straightforward as simply following the steps outlined in the Dockerfile. Building in a container is much slower than building on the local machine. So, when you build in the Debug configuration, Visual Studio actually builds your projects on the local machine, and then shares the output folder to the container using volume mounting. A build with this optimization enabled is called a Fast mode build.

In Fast mode, Visual Studio calls docker build with an argument that tells Docker to build only the base stage. Visual Studio handles the rest of the process without regard to the contents of the Dockerfile. So, when you modify your Dockerfile, such as to customize the container environment or install additional dependencies, you should put your modifications in the first stage. Any custom steps placed in the Dockerfile's build, publish, or final stages will not be executed.

This performance optimization only occurs when you build in the Debug configuration. In the Release configuration, the build occurs in the container as specified in the Dockerfile.

I put RUN touch test.txt and ENV my_variable value in the first lines (where Dockerfile build base stage) and it works.

A tip refers to RUN touch test.txt, this needs to be executed (WORKDIR) in another folder different from the folder mapped with the source code in the hostlocal (/app).

Example:

FROM mcr.microsoft.com/dotnet/core/aspnet:3.1-buster-slim AS base
WORKDIR /tmp
RUN touch teste.txt
WORKDIR /app
ENV my_variable value
EXPOSE 80
EXPOSE 443

My final Dockerfile is:

FROM mcr.microsoft.com/dotnet/core/aspnet:3.1-buster-slim AS base
WORKDIR /tmp
RUN touch teste.txt
RUN echo "teste1" > teste1.txt
WORKDIR /app
ENV my_variable value
EXPOSE 80
EXPOSE 443

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

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

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENV my_variable value
RUN touch teste.txt
ENTRYPOINT ["dotnet", "TestProject.dll"]