Add uv package manager to base image for on-demand Python support

Install uv from GitHub releases (~23MB). Users can install Python with
'uv python install 3.12' — persists across restarts via devbox-uv volume.
Eliminates need for a separate Python image variant.

Dockerfile

@@ -90,6 +90,15 @@ RUN ARCH=$(case "${TARGETARCH}" in amd64) echo "x86_64" ;; arm64) echo "aarch64"
     curl -fsSL "https://github.com/ajeetdsouza/zoxide/releases/download/v${ZOXIDE_VERSION}/zoxide-${ZOXIDE_VERSION}-${ARCH}-unknown-linux-musl.tar.gz" | tar -xz -C /usr/local/bin zoxide && \
     zoxide --version
 
+# uv — fast Python package manager (replaces pip, venv, pyenv)
+ARG UV_VERSION=0.11.6
+RUN ARCH=$(case "${TARGETARCH}" in amd64) echo "x86_64" ;; arm64) echo "aarch64" ;; *) echo "x86_64" ;; esac) && \
+    curl -fsSL "https://github.com/astral-sh/uv/releases/download/${UV_VERSION}/uv-${ARCH}-unknown-linux-musl.tar.gz" | tar -xz -C /tmp && \
+    install /tmp/uv-${ARCH}-unknown-linux-musl/uv /usr/local/bin/uv && \
+    install /tmp/uv-${ARCH}-unknown-linux-musl/uvx /usr/local/bin/uvx && \
+    rm -rf /tmp/uv-* && \
+    uv --version
+
 # Set locale
 RUN sed -i '/en_US.UTF-8/s/^# //g' /etc/locale.gen && locale-gen
 ENV LANG=en_US.UTF-8