Show and tell: .NET 8 + WASM/WASI Dockerfile

[lbussell]

Docker Desktop recently added beta support for Wasm workloads. Given that .NET supports the wasi-wasm RID, I hacked together a sample project and Dockerfile for running .NET apps using Docker Desktop's WASM runtime.

The sample is inspired by the final demo in @shanselman's recent talk at NDC Sydney (.NET and Containers - each layer of abstraction indistinguishable from magic). I improved his sample by encapsulating the build entirely within the Dockerfile instead of building the app locally and copying the .wasm bundle into the container. I also used our standard dotnetapp sample's Program.cs which provides some additional text output.

Full source: https://github.com/lbussell/dotnet-docker-wasi/

Dockerfile:

# Learn about building .NET container images:
# https://github.com/dotnet/dotnet-docker/blob/main/samples/README.md
FROM --platform=$BUILDPLATFORM mcr.microsoft.com/dotnet/sdk:8.0 AS build
ARG TARGETARCH
ARG RID="wasi-wasm"
ENV WASI_SDK_PATH=/opt/wasi-sdk/

WORKDIR /source
RUN dotnet workload install wasm-tools wasi-experimental

COPY --link . .
##
# .NET needs the WASI SDK in order to compile the project. Instead of
# downloading and extracting it, just mount the official wasi-sdk image for the
# compilation step only. The image is pinned to a specific sha since wasi-sdk
# doesn't provide stable-versioned tags.
#
# The SDK's "version check" currently looks for the existence of a VERSION* file.
# Read/write is enabled for the mounted directory so the file can be created.
# https://github.com/dotnet/runtime/blob/657f18dbd8ac9ffae3abb79104d038155387df31/src/mono/wasi/build/WasiApp.targets#L104
# https://github.com/dotnet/runtime/pull/107599#discussion_r1751918087
##
RUN --mount=type=cache,target=/root/.nuget \
    --mount=type=bind,from=ghcr.io/webassembly/wasi-sdk:sha-754aec3@sha256:919f5346107dadedddd9851db72b92d7cd52115fff3969500c9b6f3c8215cd03,source=/opt/wasi-sdk/,target=/opt/wasi-sdk/,rw=true \
    touch /opt/wasi-sdk/VERSION24 \
        && dotnet publish \
        && chmod a+x /source/bin/Release/net8.0/wasi-wasm/AppBundle/wasiconsole.wasm

FROM scratch
ENTRYPOINT ["/wasiconsole.wasm"]
COPY --link --from=build /source/bin/Release/net8.0/wasi-wasm/AppBundle/wasiconsole.wasm /wasiconsole.wasm