.NET Core - missing currency symbol in docker alpine image

During the process of moving a Scala-based API to .NET Core, we encountered an interesting localization issue when running our code in a docker container based on an alpine image. The code itself was doing a currency formatting based on some culture. It looked more or less as below

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[Route("api/[controller]")]
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[ApiController]
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public class PriceController : ControllerBase
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{
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    [HttpGet]
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    public ActionResult<object> Get(string cultureCode = "de-DE")
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    {
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        var cultureInfo = CultureInfo.CreateSpecificCulture(cultureCode);
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        var price = 10m;
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        return new
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        {
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            price,
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            formattedPrice = price.ToString("C", cultureInfo),
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            currencySymbol = cultureInfo.NumberFormat.CurrencySymbol
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        };
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    }
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}

We also had some integration tests for that piece of logic

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public class PriceTests : IClassFixture<WebApplicationFactory<Startup>>
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{
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    private readonly WebApplicationFactory<Startup> _factory;
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    public PriceTests(WebApplicationFactory<Startup> factory)
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    {
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        _factory = factory;
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    }
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    [Fact]
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    public async Task GetPrice_ReturnsCorrectCurrency()
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    {
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        var httpClient = _factory.CreateClient();
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        var httpResponseMessage = await httpClient.GetAsync("api/Price");
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        httpResponseMessage.EnsureSuccessStatusCode();
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        var content = await httpResponseMessage.Content.ReadAsStringAsync();
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        var jObject = JObject.Parse(content);
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        jObject["currencySymbol"].ToObject<string>().Should().Be("€");
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    }
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}

The tests were run during a CI build in a container with an image defined as below

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# Builder image
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FROM mcr.microsoft.com/dotnet/core/sdk:2.1.700-alpine AS builder
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WORKDIR /sln
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COPY ./*.sln ./
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# Copy the main source project files
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COPY src/*/*.csproj ./
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RUN for file in $(ls *.csproj); do mkdir -p src/${file%.*}/ && mv $file src/${file%.*}/; done
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# Copy the test project files
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COPY test/*/*.csproj ./
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RUN for file in $(ls *.csproj); do mkdir -p test/${file%.*}/ && mv $file test/${file%.*}/; done
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RUN dotnet restore
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# Copy across the rest of the source files
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COPY ./test ./test
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COPY ./src ./src
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RUN dotnet build -c Release
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RUN dotnet test "./test/AlpineMissingCurrencySymbol.Tests/AlpineMissingCurrencySymbol.Tests.csproj" \
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    -c Release --no-build --no-restore
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RUN dotnet publish "./src/AlpineMissingCurrencySymbol/AlpineMissingCurrencySymbol.csproj" \
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    -c Release -o "../../dist" --no-restore
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# App image
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FROM mcr.microsoft.com/dotnet/core/aspnet:2.1.11-alpine
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WORKDIR /app
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ENTRYPOINT ["dotnet", "AlpineMissingCurrencySymbol.dll"]
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COPY --from=builder /sln/dist .

At this point, we were sure that everything works fine, as the tests were green and everything was also working correctly on our local machines. However, after deployment to a testing environment, we started getting invalid currency symbols

curl localhost:8080/api/price
{
  "price": 10,
  "formattedPrice": "¤10.00",
  "currencySymbol": "¤"
}

As you can see the response contains ¤ (invariant currency symbol) instead of expected €. It took us some time to figure this out but finally, it turned out that aspnet:2.1.11-alpine image(the one we used for running the application) contrary to the SDK image(used for building and running tests) is missing icu-libs package. In default conditions, the application should throw the following exception during the startup

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FailFast:
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Couldn't find a valid ICU package installed on the system. Set the configuration flag System.Globalization.Invariant to true if you want to run with no globalization support.
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   at System.Environment.FailFast(System.String)
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   at System.Globalization.GlobalizationMode.GetGlobalizationInvariantMode()
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   at System.Globalization.GlobalizationMode..cctor()
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   at System.Globalization.CultureData.CreateCultureWithInvariantData()
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   at System.Globalization.CultureData.get_Invariant()
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   at System.Globalization.CultureInfo..cctor()
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   at System.StringComparer..cctor()
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   at System.AppDomain.InitializeCompatibilityFlags()
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   at System.AppDomain.Setup(System.Object)

However, aspnet:2.1.11-alpine image has the DOTNET_SYSTEM_GLOBALIZATION_INVARIANT flag set to true by default, so the missing package was not validated during a startup. After all, in order to fix the issue, we had to install the icu-libs package and also set the DOTNET_SYSTEM_GLOBALIZATION_INVARIANT back to false. This was done by these two lines in Dockerfile

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ENV DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=false
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RUN apk add --no-cache icu-libs

Once the lines were added the application started working as expected

curl localhost:8080/api/price
{
  "price": 10,
  "formattedPrice": "10,00 €",
  "currencySymbol": "€"
}

Source code for this post can be found here