Allow customization of API server user via build arg

Add API_USER build argument to Dockerfile so the API server execution
user can be customized at build time instead of being hardcoded as
'piston'. This helps users who want to self-host without exposing
the 'piston' name. Default remains 'piston' for backward compatibility.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: koki-develop

api/Dockerfile

@@ -10,6 +10,7 @@ RUN apt-get update && \
 
 FROM node:20.19.6-bookworm-slim@sha256:a270640213033657548b7ed57852709a0d2ee3b277f902f912367216aa8e9520
 
+ARG API_USER=piston
 ENV DEBIAN_FRONTEND=noninteractive
 
 RUN dpkg-reconfigure -p critical dash
@@ -21,7 +22,8 @@ RUN apt-get update && \
     libfftw3-dev libglpk-dev libqhull-dev libqrupdate-dev libsuitesparse-dev \
     libsundials-dev libpcre2-dev libcap-dev && \
     rm -rf /var/lib/apt/lists/*
-RUN useradd -M piston
+RUN useradd -M ${API_USER}
+ENV API_USER=${API_USER}
 COPY --from=isolate /usr/local/bin/isolate /usr/local/bin
 COPY --from=isolate /usr/local/etc/isolate /usr/local/etc/isolate
 

api/src/docker-entrypoint.sh

@@ -25,5 +25,5 @@ mkdir init && \
 echo 1 > init/cgroup.procs && \
 echo '+cpuset +memory' > cgroup.subtree_control && \
 echo "Initialized cgroup" && \
-chown -R piston:piston /piston && \
-exec su -- piston -c 'ulimit -n 65536 && node /piston_api/src'
+chown -R "${API_USER}:${API_USER}" /piston && \
+exec su -- "${API_USER}" -c 'ulimit -n 65536 && node /piston_api/src'