feat: add custom sandbox Dockerfile with sudo support

- Add Dockerfile.sandbox with sudo, writable passwd/shadow, and
  entrypoint that registers dynamic UIDs for full root access inside container
- Remove playwright MCP server (npx not available in sandbox)
- Move sandbox host_commands/image config to global workmux config
- Remove git from host_commands to prevent infinite fork bomb via shims

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.mcp.json

@@ -3,10 +3,6 @@
     "svelte": {
       "type": "http",
       "url": "https://mcp.svelte.dev/mcp"
-    },
-    "playwright": {
-      "command": "npx",
-      "args": ["@playwright/mcp@latest"]
     }
   }
 }

.workmux.yaml

@@ -45,4 +45,4 @@ files:
 sandbox:
   enabled: false
   toolchain: off
-  host_commands: ["cargo", "npm", "npx", "node", "git"]
+  # image and host_commands configured in global ~/.config/workmux/config.yaml

Dockerfile.sandbox

@@ -0,0 +1,127 @@
+FROM debian:bookworm-slim
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    curl \
+    ca-certificates \
+    git \
+    iptables \
+    gosu \
+    sudo \
+    && rm -rf /var/lib/apt/lists/* \
+    && echo "ALL ALL=(ALL) NOPASSWD: ALL" > /etc/sudoers.d/sandbox \
+    && chmod 0440 /etc/sudoers.d/sandbox \
+    && chmod 666 /etc/passwd /etc/group /etc/shadow
+
+# Embed network init script (sets up iptables firewall, then drops privileges)
+RUN cat <<'SCRIPT' > /usr/local/bin/network-init.sh
+#!/bin/bash
+set -euo pipefail
+
+if [ -n "${WM_PROXY_HOST:-}" ] && [ -n "${WM_PROXY_PORT:-}" ]; then
+    # Resolve hostnames to ALL IPs (multi-A records, round-robin DNS)
+    PROXY_IPS=$(getent ahostsv4 "$WM_PROXY_HOST" | awk '{print $1}' | sort -u)
+    RPC_HOST="${WM_RPC_HOST:-$WM_PROXY_HOST}"
+    RPC_IPS=$(getent ahostsv4 "$RPC_HOST" | awk '{print $1}' | sort -u)
+
+    if [ -z "$PROXY_IPS" ] || [ -z "$RPC_IPS" ]; then
+        echo "network-init: failed to resolve proxy/RPC host" >&2
+        exit 1
+    fi
+
+    # IPv4: default deny outbound
+    iptables -P OUTPUT DROP
+    iptables -A OUTPUT -o lo -j ACCEPT
+    iptables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
+
+    # Allow DNS (UDP/TCP 53) to configured nameservers.
+    # Without this, any hostname resolution hangs until timeout.
+    if [ -f /etc/resolv.conf ]; then
+        grep '^nameserver' /etc/resolv.conf | awk '{print $2}' | while read -r ns; do
+            iptables -A OUTPUT -d "$ns" -p udp --dport 53 -j ACCEPT
+            iptables -A OUTPUT -d "$ns" -p tcp --dport 53 -j ACCEPT
+        done
+    fi
+
+    # Allow ALL resolved proxy IPs (handles multi-A DNS)
+    for ip in $PROXY_IPS; do
+        iptables -A OUTPUT -d "$ip" -p tcp --dport "$WM_PROXY_PORT" -j ACCEPT
+    done
+
+    # Allow ALL resolved RPC IPs
+    if [ -n "${WM_RPC_PORT:-}" ]; then
+        for ip in $RPC_IPS; do
+            iptables -A OUTPUT -d "$ip" -p tcp --dport "$WM_RPC_PORT" -j ACCEPT
+        done
+    fi
+
+    # Reject (not drop) everything else to fail fast instead of hanging
+    iptables -A OUTPUT -j REJECT
+
+    # IPv6: block entirely to prevent leaks (fail closed)
+    if ip6tables -L -n >/dev/null 2>&1; then
+        ip6tables -P OUTPUT DROP
+        ip6tables -A OUTPUT -o lo -j ACCEPT
+        ip6tables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
+        ip6tables -A OUTPUT -j REJECT
+    else
+        # If ip6tables unavailable, disable IPv6 via sysctl as fallback
+        if ! sysctl -w net.ipv6.conf.all.disable_ipv6=1 2>/dev/null; then
+            echo "network-init: failed to block IPv6 (neither ip6tables nor sysctl available)" >&2
+            exit 1
+        fi
+    fi
+fi
+
+# Add sandbox user to passwd so sudo works with arbitrary UIDs.
+if ! getent passwd "${WM_TARGET_UID}" >/dev/null 2>&1; then
+    echo "sandbox:x:${WM_TARGET_UID}:${WM_TARGET_GID}:sandbox:/tmp:/bin/sh" >> /etc/passwd
+    echo "sandbox:x:${WM_TARGET_GID}:" >> /etc/group 2>/dev/null || true
+fi
+
+# Fix PTY ownership so the unprivileged user can read/write the terminal.
+# Docker allocates the PTY as root; after gosu drops privileges, the user
+# has no access to /dev/pts/0 unless we transfer ownership.
+if [ -t 0 ]; then
+    chown "${WM_TARGET_UID}:${WM_TARGET_GID}" "$(tty)"
+fi
+
+# Drop privileges and exec the user command.
+# gosu resets HOME via getpwuid() which returns "/" for UIDs not in /etc/passwd.
+# Preserve the container's HOME=/tmp so agents find their config dirs.
+exec gosu "${WM_TARGET_UID}:${WM_TARGET_GID}" env HOME=/tmp "$@"
+SCRIPT
+RUN chmod +x /usr/local/bin/network-init.sh
+
+# Install workmux (needed for sandbox RPC)
+RUN curl -fsSL https://raw.githubusercontent.com/raine/workmux/main/scripts/install.sh | bash
+
+# --- claude agent ---
+# Install Claude Code, then move to a globally accessible path so the binary
+# is reachable when running as an arbitrary non-root UID (--user uid:gid).
+RUN curl -fsSL https://claude.ai/install.sh | bash && \
+    target="$(readlink -f /root/.local/bin/claude)" && \
+    mv /root/.local/share/claude /opt/claude && \
+    ln -s "/opt/claude/versions/$(basename "$target")" /usr/local/bin/claude && \
+    # Claude Code checks $HOME/.local/bin/claude when installMethod is "native".
+    # Container sets HOME=/tmp, so place a symlink to suppress the warning.
+    mkdir -p /tmp/.local/bin && \
+    ln -s /usr/local/bin/claude /tmp/.local/bin/claude && \
+    chmod 777 /tmp/.local
+
+# Entrypoint that registers the dynamic UID in /etc/passwd so sudo works.
+# workmux passes --user UID:GID directly, bypassing network-init.sh.
+RUN cat <<'ENTRY' > /usr/local/bin/entrypoint.sh
+#!/bin/sh
+if ! getent passwd "$(id -u)" >/dev/null 2>&1; then
+    echo "sandbox:x:$(id -u):$(id -g):sandbox:/tmp:/bin/sh" >> /etc/passwd
+fi
+if ! getent group "$(id -g)" >/dev/null 2>&1; then
+    echo "sandbox:x:$(id -g):" >> /etc/group
+fi
+if ! grep -q "^sandbox:" /etc/shadow 2>/dev/null; then
+    echo "sandbox:*:19000:0:99999:7:::" >> /etc/shadow
+fi
+exec "$@"
+ENTRY
+RUN chmod +x /usr/local/bin/entrypoint.sh
+ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]