Run agent-browser install as coder user via runuser

Use `runuser -u coder -- agent-browser install` in the root RUN block
so Chromium is installed into the coder user's home directory in-place,
instead of a separate RUN layer after USER coder.

https://claude.ai/code/session_01MukmmJhsNvpwRmVAqAQvd8

Author: claude

fx/Dockerfile

@@ -50,7 +50,7 @@ RUN apt update && apt dist-upgrade -y && \
         libxi6 libgtk-3-0t64 libcairo2 libcairo-gobject2 libgdk-pixbuf-2.0-0 \
         libatk1.0-0 libxrender1 libfontconfig1 libdbus-1-3 libxcb1 libxext6 libx11-6 && \
     npm install -g agent-browser && \
-    agent-browser install && \
+    runuser -u coder -- agent-browser install && \
     # PROFILE (DENO, BUN PATH) ##################################################
     echo '' >> /etc/profile && \
     echo '# Deno, Bun' >> /etc/profile && \
@@ -68,5 +68,3 @@ RUN apt update && apt dist-upgrade -y && \
 WORKDIR /home/coder
 
 USER coder
-
-RUN agent-browser install