Passing Environment Variables From Docker to .NET Web App

We have a build pipeline running in GitHub Actions that builds a .NET 6 web in a Docker container and then publishes it to AWS. Within this pipeline we have a manual step where we can chose the source branch or tag and the environment it should be deployed to (dev/stg/prd). In order to make it more obvious what has been deployed where we were looking to pass the branch/tag name through to our web app where it could be displayed on the Swagger page.

Although this is pretty straightforward it took a while to figure out all the steps required.

First we need to pass the github.ref_name through as a build argument to our Docker container. For some reason this only worked for me when it was double quoted and done as a multi-line input.

      - name: Build and push
        id: buildpush
        uses: docker/build-push-action@v2
        if: ${{ steps.setup.outputs.imageExists == 'false' }}
        with:
          push: true
          secrets: ${{ fromJson(steps.setup.outputs.dockerParams).secrets }}
          build-args: |
            "DEPLOYMENT_SOURCE=${{ github.ref_name }}"
          tags: ${{ fromJson(steps.setup.outputs.dockerParams).tags }}
          context: ${{ fromJson(steps.setup.outputs.dockerParams).context }}
          file: ./MyApi/Dockerfile

This then passes a build argument called DEPLOYMENT_SOURCE through to the Dockerfile, within the Dockerfile this then needs to be set as an environment variable.

FROM base AS final
ARG DEPLOYMENT_SOURCE
ENV Swagger__DeploymentSource ${DEPLOYMENT_SOURCE}
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "./MyApi.Api.dll"] 

This maps the build argument DEPLOYMENT_SOURCE to an environment variable called Swagger__DeploymentSource, this will then overwrite the value Swagger.DeploymentSource in our appsettings.json file.

private OpenApiInfo CreateVersionInfo(ApiVersionDescription description)
{
    var info = new OpenApiInfo()
    {
        Title = _swaggerOptions.Title,
        Description = $"Deployment Source: {_swaggerOptions.DeploymentSource}</br>Commit: {Environment.GetEnvironmentVariable("BOSUN_IMAGE_TAG")}",
        Version = description.ApiVersion.ToString()
    };

    if (description.IsDeprecated)
    {
        info.Description += " This API version has been deprecated.";
    }

    return info;
}