Add messy Dockerfile for building twelf wasm dependencies

My goal is to set down once and for all a reproducible build process
that shows how the `twelf.wasm` was constructed, instead of it
existing ad hoc across my memory and comments on the
PR MLton/mlton#550 . Already I discovered a
couple of crucial omissions in those comments.

This still deserves some cleanup, as the ordering of steps is not
really a coherent narrative, because I added some as I discovered
them. A cleaner one is forthcoming.

docker/Dockerfile

@@ -0,0 +1,76 @@
+FROM ubuntu:latest
+
+RUN mkdir /build
+WORKDIR /build
+RUN apt-get update
+RUN apt-get install -y git
+
+# wasi-sdk
+RUN git clone --recursive https://github.com/WebAssembly/wasi-sdk.git && \
+    cd wasi-sdk && \
+	 apt-get install -y cmake clang ninja-build && \
+	 NINJA_FLAGS=-v make package
+
+# XXX move up eventually
+ENV BUILD=/build
+RUN apt-get install -y wget xz-utils
+
+# gmp
+RUN wget https://gmplib.org/download/gmp/gmp-6.3.0.tar.xz && \
+    tar xvf gmp-6.3.0.tar.xz
+WORKDIR $BUILD/gmp-6.3.0
+# XXX get rid of this WORKDIR ^
+
+ENV WASISDK=$BUILD/wasi-sdk/build/install/opt/wasi-sdk
+
+RUN ./configure --host=wasm32-unknown-wasi \
+		  CC=$WASISDK/bin/clang \
+		  RANLIB=$WASISDK/bin/ranlib \
+		  CFLAGS=-D_WASI_EMULATED_SIGNAL LDFLAGS=-lwasi-emulated-signal \
+		  --prefix=$BUILD/gmp-wasi-INSTALL && \
+    make
+
+# mlton binaries (for bootstrap)
+WORKDIR $BUILD
+RUN wget https://github.com/MLton/mlton/releases/download/on-20210117-release/mlton-20210117-1.amd64-linux-glibc2.31.tgz && \
+    tar xvzf mlton-20210117-1.amd64-linux-glibc2.31.tgz && \
+    cd $BUILD/mlton-20210117-1.amd64-linux-glibc2.31 && make install
+
+# mlton at branch agoode/wasm2, which at time of writing was commit hash d2b9e5d3ea
+RUN git clone -b wasm2 https://github.com/agoode/mlton mlton
+
+# XXX move up
+RUN apt-get install -y libgmp-dev
+
+# XXX oops get rid of this ls below
+
+# build mlton binary
+RUN ls && \
+    cd mlton && \
+    export MLTON=$BUILD/mlton-INSTALL && \
+    make all && \
+    make PREFIX=$MLTON install
+
+# XXX move up
+RUN apt-get -y install curl
+
+# install wasmtime, XXX move up
+RUN curl https://wasmtime.dev/install.sh -sSf | bash
+ENV PATH="/root/.wasmtime/bin:$PATH"
+
+# XXX move this up
+RUN cd $BUILD/gmp-6.3.0 && \
+    make install
+
+# build wasm runtime
+RUN cd mlton && make clean && \
+    make \
+		  CC=$WASISDK/bin/clang \
+		  AR=$WASISDK/bin/ar \
+		  RANLIB=$WASISDK/bin/ranlib \
+		  TARGET_OS=wasi \
+		  TARGET_ARCH=wasm32 \
+		  TARGET=wasm32-unknown-wasi \
+		  WITH_GMP_DIR=$BUILD/gmp-wasi-INSTALL \
+		  PREFIX=$BUILD/mlton-INSTALL \
+		  dirs runtime install-runtime