How do I get a connection string into a docker container in heroku securely?

Question

I've been losing my mind trying to find a way to securely get a connection string into a docker container when using Heroku. This is what the docs told me:

"We suggest using ENV for runtime variables (eg, GEM_PATH) and heroku config for credentials, so that sensitive credentials aren't accidentally checked into source code control."

In this case for a connection string I'd want to set a variable via heroku config, thats what I did. Hit this trying to use heroku config:edit, so I'm using heroku config --json.

This variable is in there:

  "ConnectionStrings__Default": "MyConnectionString"

Now as for how to pass this in the actual dockerfile I'm not good with docker so a lot of this is guess work in combination with what I found about externally getting connection strings into c# here . I am suspecting the problem is somewhere around here so please do tell me if this is incorrect:

# publish
FROM build AS publish
WORKDIR /src/webapplication/webapplication
RUN dotnet publish -c Release -o /src/publish
FROM mcr.microsoft.com/dotnet/core/aspnet:3.0 AS runtime
WORKDIR /app
COPY --from=publish /src/publish .
# heroku uses the following
CMD Default=$ConnectionStrings__Default ASPNETCORE_URLS=http://*:$PORT dotnet webapplication.dll

Am I correct in inserting a configuration variable into an aspnet core application in docker this way? I could find very little examples of this so not sure.

The code getting the connection string (works when using json.config):

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            string basePath = Directory.GetCurrentDirectory();
            if (!optionsBuilder.IsConfigured)
            {
                var builder = new ConfigurationBuilder()
       .SetBasePath(basePath)
       .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true);

                IConfigurationRoot configuration = builder.Build();

                string connectionString = configuration.GetConnectionString("Default");
                optionsBuilder.UseSqlServer(connectionString)
                    .EnableSensitiveDataLogging();
                optionsBuilder.UseLazyLoadingProxies();

            }
        }

The error:

System.ArgumentNullException: Value cannot be null. (Parameter 'connectionString')

If anyone could give me some pointers it would mean the world to me because I want to deploy this thing but I have no idea what to do.

Answer 1

After .AddJsonFile() you need to call .AddEnvironmentVariables() which will overwrite any settings from your settings file with environment variables that match.

Your code should look like this

var builder = new ConfigurationBuilder()
    .SetBasePath(basePath)
    .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
    .AddEnvironmentVariables();

Another option is to use the -e flag when calling docker run to set environment variables in the container

https://docs.docker.com/engine/reference/run/#env-environment-variables