Added multi-stage build process and documentation

Dockerfile

@@ -1,10 +1,34 @@
-FROM ubuntu:23.10
+# The Dockerfile consists of four stages, each setting up components for a 
+# RISC-V development environment.
+#
+# - Stage 1: Utilises Ubuntu 23.10 to install build tools and dependencies, 
+#            then clones, compiles, and installs the RISC-V GNU toolchain in 
+#            `/opt/riscv`.
+# 
+# - Stage 2: Sets up another Ubuntu 23.10 environment, copies the RISC-V 
+#            toolchain from Stage 1, installs additional dependencies, and then 
+#            clones, compiles, and installs the RISC-V Proxy Kernel.
+# 
+# - Stage 3: Uses Ubuntu 23.10 again, brings forward tools from previous 
+#            stages, installs further dependencies, and then clones, compiles, 
+#            and installs the Spike RISC-V ISA simulator.
+#
+# - Stage 4: Prepares the final Ubuntu 23.10 environment, copies all tools, 
+#            updates the PATH to include `/opt/riscv/bin`, and configures the 
+#            container to start with a shell for immediate interaction.
+
+############################## Stage 1
+
+FROM ubuntu:23.10 AS stage1
 
 ENV DEBIAN_FRONTEND=noninteractive
 
 WORKDIR /root
 
-# Install prerequisites
+# RISC-V toolchain
+# Source: https://github.com/riscv-collab/riscv-gnu-toolchain
+
+## Install prerequisites
 RUN apt update -qq && apt install -y --no-install-recommends \
     autoconf \
     automake \
@@ -33,7 +57,7 @@ RUN apt update -qq && apt install -y --no-install-recommends \
     libslirp-dev \
     && rm -rf /var/lib/apt/lists/*
 
-# Compile and install toolchain
+## Compile and install
 RUN git clone https://github.com/riscv/riscv-gnu-toolchain
 
 WORKDIR /root/riscv-gnu-toolchain
@@ -45,12 +69,105 @@ WORKDIR /root
 
 RUN rm -rf /root/riscv-gnu-toolchain
 
-# Install additional prerequisites
+############################## Stage 2
+
+FROM ubuntu:23.10 AS stage2
+
+COPY --from=stage1 /opt/riscv /opt/riscv
+
+WORKDIR /root
+
+## Install prerequisites
 RUN apt update -qq && apt install -y --no-install-recommends \
+    autoconf \
+    automake \
+    autotools-dev \
+    curl \
+    python3 \
+    python3-pip \
+    libmpc-dev \
+    libmpfr-dev \
+    libgmp-dev \
+    gawk \
+    build-essential \
+    bison \
+    flex \
+    texinfo \
+    gperf \
+    libtool \
+    patchutils \
+    bc \
+    zlib1g-dev \
+    libexpat-dev \
+    ninja-build \
+    git \
+    cmake \
+    libglib2.0-dev \
+    libslirp-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+# RISC-V proxy kernel and boot loader
+# Source: https://github.com/riscv-software-src/riscv-pk
+
+## Compile and install
+RUN git clone https://github.com/riscv-software-src/riscv-pk
+
+WORKDIR /root/riscv-pk
+
+ENV PATH="${PATH}:/opt/riscv/bin/"
+
+RUN mkdir build && \
+    cd build && \
+    ../configure --prefix=/opt/riscv --host=riscv64-unknown-elf --with-arch=rv64gc_zifencei && \
+    make -j8 && \
+    make install
+
+WORKDIR /root
+
+RUN rm -rf /root/riscv-pk
+
+############################## Stage 3
+
+FROM ubuntu:23.10 AS stage3
+
+COPY --from=stage2 /opt/riscv /opt/riscv
+
+WORKDIR /root
+
+## Install prerequisites
+RUN apt update -qq && apt install -y --no-install-recommends \
+    autoconf \
+    automake \
+    autotools-dev \
+    curl \
+    python3 \
+    python3-pip \
+    libmpc-dev \
+    libmpfr-dev \
+    libgmp-dev \
+    gawk \
+    build-essential \
+    bison \
+    flex \
+    texinfo \
+    gperf \
+    libtool \
+    patchutils \
+    bc \
+    zlib1g-dev \
+    libexpat-dev \
+    ninja-build \
+    git \
+    cmake \
+    libglib2.0-dev \
+    libslirp-dev \
     device-tree-compiler \
     && rm -rf /var/lib/apt/lists/*
 
-# Compile and install Spike
+# Spike, RISC-V ISA simulator
+# Source: https://github.com/riscv-software-src/riscv-isa-sim
+
+## Compile and install
 RUN git clone https://github.com/riscv/riscv-isa-sim.git
 
 WORKDIR /root/riscv-isa-sim
@@ -65,5 +182,15 @@ WORKDIR /root
 
 RUN rm -rf /root/riscv-isa-sim
 
+############################## Stage 4
+
+FROM ubuntu:23.10 AS stage4
+
+COPY --from=stage3 /opt/riscv /opt/riscv
+
+WORKDIR /root
+
+RUN echo "export PATH=$PATH:/opt/riscv/bin" >> /root/.bashrc
+
 # Pass commands into container
 ENTRYPOINT ["bash", "-lc"]