diff --git a/.dockerignore b/.dockerignore
index 4042bca..bdcb1e7 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -1,7 +1,5 @@
-.git
-.gitignore
-Dockerfile*
-docker-compose*
-README.md
-LICENSE
-.vscode
\ No newline at end of file
+# Only the two files the image COPYs belong in the build context.
+# Everything else (scripts, docs, .git, the broken puffertank symlink) is noise.
+*
+!entrypoint.sh
+!init.vim
diff --git a/puffertank.dockerfile b/puffertank.dockerfile
index 7badb7e..3b1c75e 100644
--- a/puffertank.dockerfile
+++ b/puffertank.dockerfile
@@ -1,73 +1,100 @@
+# syntax=docker/dockerfile:1
+
+# ---------------------------------------------------------------------------
+# Stage 0: build Neovim from source (latest master). The final image only
+# copies the installed runtime (/opt/nvim) -- not cmake/ninja/gettext, the
+# source tree, or the build artifacts.
+# ---------------------------------------------------------------------------
+FROM nvcr.io/nvidia/cuda:13.0.2-cudnn-devel-ubuntu24.04 AS nvim-builder
+ARG DEBIAN_FRONTEND=noninteractive
+ARG NEOVIM_REF=master
+
+RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
+    --mount=type=cache,target=/var/lib/apt,sharing=locked \
+    apt-get update \
+    && apt-get install -y --no-install-recommends ninja-build gettext cmake unzip
+
+RUN git clone --single-branch --depth=1 --branch ${NEOVIM_REF} https://github.com/neovim/neovim \
+    && cd neovim \
+    && make CMAKE_BUILD_TYPE=Release CMAKE_INSTALL_PREFIX=/opt/nvim -j$(nproc) \
+    && make install
+
+# ---------------------------------------------------------------------------
+# Stage 1: the dev container. Layers are ordered from least-frequently to
+# most-frequently changed, so expensive work (apt, nvim, torch, pufferlib)
+# stays cached while config files churn. Cache mounts survive rebuilds.
+# ---------------------------------------------------------------------------
 FROM nvcr.io/nvidia/cuda:13.0.2-cudnn-devel-ubuntu24.04
 ARG DEBIAN_FRONTEND=noninteractive
 
-RUN mkdir -p /puffertank
 WORKDIR /puffertank
 
-# Core system packages
-# Custom installs without the cudnn base also need libnccl2 libnccl-dev
-RUN apt-get update && apt-get install -y curl wget sudo git build-essential clang
+# Core system packages.
+# Custom installs without the cudnn base also need libnccl2 libnccl-dev.
+RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
+    --mount=type=cache,target=/var/lib/apt,sharing=locked \
+    apt-get update \
+    && apt-get install -y curl wget sudo git build-essential clang \
+        htop gdb tmux psmisc llvm ccache \
+        sqlite3 \
+        libomp-dev libglfw3 libgl1-mesa-dev python3.12-dev
 
-# UV venv
+# Nsight Systems for profiling (own layer: large, and version-bumped on its own).
+RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
+    --mount=type=cache,target=/var/lib/apt,sharing=locked \
+    apt-get update \
+    && apt-get install -y --no-install-recommends nsight-systems-2025.6.3
+
+# UV venv + pynvim
 RUN curl -LsSf https://astral.sh/uv/install.sh | sh \
     && . $HOME/.local/bin/env \
-    && uv venv --python 3.12 --prompt 🐡 venv
-
-# Neovim (btw)
-RUN . $HOME/.local/bin/env \
+    && uv venv --python 3.12 --prompt 🐡 venv \
     && . venv/bin/activate \
-    && apt-get install -y ninja-build gettext cmake unzip \
-    && git clone --single-branch --depth=1 https://github.com/neovim/neovim \
-    && cd neovim \
-    && make CMAKE_BUILD_TYPE=Release \
-    && make install \
-    && ln -s /usr/local/bin/nvim /usr/bin/nvim \
-    && . $HOME/.local/bin/env \
-    && uv pip install pynvim \
-    && sh -c 'curl -fLo "${XDG_DATA_HOME:-$HOME/.local/share}"/nvim/site/autoload/plug.vim --create-dirs https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim'
-# My personal config
-COPY init.vim /root/.config/nvim/init.vim
-
-RUN apt-get install -y\
-    htop gdb tmux psmisc llvm ccache \
-    sqlite3 \
-    libomp-dev libglfw3 libgl1-mesa-dev python3.12-dev 
+    && uv pip install pynvim
 
-# Nsight Systems for profiling
-RUN apt-get install -y --no-install-recommends nsight-systems-2025.6.3
+# Neovim runtime, built in the stage above
+COPY --from=nvim-builder /opt/nvim /opt/nvim
+RUN ln -s /opt/nvim/bin/nvim /usr/bin/nvim \
+    && sh -c 'curl -fLo "${XDG_DATA_HOME:-$HOME/.local/share}"/nvim/site/autoload/plug.vim --create-dirs https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim'
 
-# PyTorch + uv env
-RUN . $HOME/.local/bin/env \
+# PyTorch. The uv cache mount persists across builds: ~3GB of wheels download once.
+RUN --mount=type=cache,target=/root/.cache/uv \
+    . $HOME/.local/bin/env \
     && . venv/bin/activate \
     && uv pip install torch --index-url https://download.pytorch.org/whl/cu130
 
-# PufferLib + docs + 20k baseline experiments viewable with Constellation
-RUN --mount=type=cache,target=/root/.ccache . $HOME/.local/bin/env \
-    && git clone https://github.com/pufferai/pufferlib --branch 4.0 \
-    && git clone https://github.com/pufferai/puffer.ai --branch 4.0 \
+# PufferLib + docs + 20k baseline experiments viewable with Constellation.
+# Bump PUFFERLIB_REF / PUFFERAI_REF (or pass --build-arg) to refresh this layer.
+ARG PUFFERLIB_REF=4.0
+ARG PUFFERAI_REF=4.0
+RUN --mount=type=cache,target=/root/.ccache \
+    --mount=type=cache,target=/root/.cache/uv \
+    --mount=type=cache,target=/root/.cache/puffer \
+    . $HOME/.local/bin/env \
+    && git clone https://github.com/pufferai/pufferlib --branch ${PUFFERLIB_REF} \
+    && git clone https://github.com/pufferai/puffer.ai --branch ${PUFFERAI_REF} \
     && . venv/bin/activate \
     && cd pufferlib \
     && uv pip install -e . \
     && bash build.sh breakout \
-    && curl -L -o experiments.zip https://github.com/PufferAI/PufferLib/releases/download/experiments/experiments.zip \
-    && unzip -q experiments.zip \
-    && rm experiments.zip \
+    && curl -L -o /root/.cache/puffer/experiments.zip https://github.com/PufferAI/PufferLib/releases/download/experiments/experiments.zip \
+    && unzip -q /root/.cache/puffer/experiments.zip \
     && python constellation/cache_data.py --full \
     && bash build.sh constellation --fast
 
-# Run on container startup
-COPY entrypoint.sh /root/entrypoint.sh
-RUN chmod +x /root/entrypoint.sh
-ENTRYPOINT ["/root/entrypoint.sh"]
-
 # Bashrc
 RUN echo "export PS1=$''" >> ~/.bashrc \
- && echo "alias vim='/usr/bin/nvim'" >> ~/.bashrc \ 
- && echo "alias diff='diff --color --palette=':ad=36:de=31:ln=33''" >> ~/.bashrc \
- && echo "alias pip='uv pip'" >> ~/.bashrc \
- && echo ". /puffertank/venv/bin/activate" >> ~/.bashrc \
- && echo "cd /puffertank/pufferlib" >> ~/.bashrc \
- && echo "export __GLX_VENDOR_LIBRARY_NAME=mesa" >> ~/.bashrc
+    && echo "alias vim='/usr/bin/nvim'" >> ~/.bashrc \
+    && echo "alias diff='diff --color --palette=':ad=36:de=31:ln=33''" >> ~/.bashrc \
+    && echo "alias pip='uv pip'" >> ~/.bashrc \
+    && echo ". /puffertank/venv/bin/activate" >> ~/.bashrc \
+    && echo "cd /puffertank/pufferlib" >> ~/.bashrc \
+    && echo "export __GLX_VENDOR_LIBRARY_NAME=mesa" >> ~/.bashrc
+
+# Config files last: the only COPY steps, so nothing above them is invalidated.
+COPY --chmod=755 entrypoint.sh /root/entrypoint.sh
+COPY init.vim /root/.config/nvim/init.vim
 
-RUN apt-get clean
+# Run on container startup
+ENTRYPOINT ["/root/entrypoint.sh"]
 CMD ["/bin/bash"]