(reproducibility) add Dockerfile

I had some difficulty setting up the C toolchains, here's everything pinned down as a sanity check

```sh
docker build -t example-c-host .
docker run example-c-host
```

Co-authored-by: Shelley <shelley@exe.dev>

Author: 2 people

component-model/examples/example-c-host/Dockerfile

@@ -0,0 +1,58 @@
+FROM ubuntu:24.04 AS build
+
+ARG WASMTIME_VERSION=42.0.1
+ARG WASI_SDK_VERSION=27
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    gcc libc6-dev curl xz-utils ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+# Install wasmtime C API
+RUN curl -sL https://github.com/bytecodealliance/wasmtime/releases/download/v${WASMTIME_VERSION}/wasmtime-v${WASMTIME_VERSION}-x86_64-linux-c-api.tar.xz \
+    | tar xJ --strip-components=1 -C /usr/local \
+    && ldconfig
+
+# Install wasi-sdk (for building the guest component)
+RUN curl -sLO https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-${WASI_SDK_VERSION}/wasi-sdk-${WASI_SDK_VERSION}.0-x86_64-linux.deb \
+    && dpkg -i wasi-sdk-${WASI_SDK_VERSION}.0-x86_64-linux.deb \
+    && rm wasi-sdk-${WASI_SDK_VERSION}.0-x86_64-linux.deb
+
+# Install wit-bindgen and wasm-tools (pin versions)
+ARG WIT_BINDGEN_VERSION=0.53.1
+ARG WASM_TOOLS_VERSION=1.245.1
+RUN curl -sL https://github.com/bytecodealliance/wit-bindgen/releases/download/v${WIT_BINDGEN_VERSION}/wit-bindgen-${WIT_BINDGEN_VERSION}-x86_64-linux.tar.gz \
+    | tar xz --strip-components=1 -C /usr/local/bin --wildcards '*/wit-bindgen' \
+    && curl -sL https://github.com/bytecodealliance/wasm-tools/releases/download/v${WASM_TOOLS_VERSION}/wasm-tools-${WASM_TOOLS_VERSION}-x86_64-linux.tar.gz \
+    | tar xz --strip-components=1 -C /usr/local/bin --wildcards '*/wasm-tools'
+
+WORKDIR /src
+
+# Fetch guest source from upstream component-docs
+ARG COMPONENT_DOCS_REV=7c1b5a9
+RUN curl -sL -o world.wit \
+      https://raw.githubusercontent.com/bytecodealliance/component-docs/${COMPONENT_DOCS_REV}/component-model/examples/tutorial/wit/adder/world.wit \
+    && mkdir -p wit/adder && mv world.wit wit/adder/world.wit
+RUN curl -sL -o component.c \
+      https://raw.githubusercontent.com/bytecodealliance/component-docs/${COMPONENT_DOCS_REV}/component-model/examples/tutorial/c/adder/component.c
+
+# Build the guest component
+RUN wit-bindgen c wit/adder/world.wit
+RUN /opt/wasi-sdk/bin/wasm32-wasip2-clang \
+    -o adder.wasm \
+    -mexec-model=reactor \
+    component.c adder.c adder_component_type.o
+
+# Build the host
+COPY host.c .
+RUN gcc -o /usr/local/bin/adder-host host.c -lwasmtime
+
+# Runtime image
+FROM ubuntu:24.04
+
+COPY --from=build /usr/local/lib/libwasmtime.so /usr/local/lib/
+RUN ldconfig
+COPY --from=build /usr/local/bin/adder-host /usr/local/bin/adder-host
+COPY --from=build /src/adder.wasm /opt/adder.wasm
+
+ENTRYPOINT ["adder-host"]
+CMD ["1", "2", "/opt/adder.wasm"]