Dev container: fix arm64 build (arch-aware Chrome) and pin postgres:16

.devcontainer/Dockerfile

@@ -8,6 +8,14 @@
 FROM mcr.microsoft.com/devcontainers/ruby:dev-3.4-bookworm
 RUN export DEBIAN_FRONTEND=noninteractive
 RUN apt-get update && apt-get -y install vim curl gpg postgresql postgresql-contrib
-RUN cd /tmp
-RUN wget https://dl.google.com/linux/direct/google-chrome-stable_current_amd64.deb \
-    && apt-get -y install ./google-chrome-stable_current_amd64.deb
+# Headless browser for Cuprite/Capybara system specs. Google Chrome only ships
+# an amd64 .deb, so on arm64 (Apple Silicon hosts) install Debian's Chromium
+# instead, which Ferrum/Cuprite auto-detects. On amd64 keep upstream Chrome.
+RUN set -eux; \
+    arch="$(dpkg --print-architecture)"; \
+    if [ "$arch" = "amd64" ]; then \
+      cd /tmp && wget https://dl.google.com/linux/direct/google-chrome-stable_current_amd64.deb \
+        && apt-get -y install ./google-chrome-stable_current_amd64.deb; \
+    else \
+      apt-get -y install chromium; \
+    fi

.devcontainer/docker-compose.yml

@@ -20,7 +20,11 @@ services:
     network_mode: service:postgres
 
   postgres:
-    image: postgres:latest
+    # Pin a stable major version. `postgres:latest` is now PG18, whose Docker
+    # image refuses to start against the legacy /var/lib/postgresql/data mount
+    # layout this compose file uses (it wants the mount one level up). Pinning
+    # avoids that and keeps the DB version reproducible across hosts.
+    image: postgres:16
     restart: always
     volumes:
       - postgres-data:/var/lib/postgresql/data