Generate all UTF-8 locales, allow locale override via env vars

Users can set LANG, LANGUAGE, LC_ALL in .env to override the default
en_US.UTF-8 locale (e.g. sv_SE.UTF-8 for Swedish).

Dockerfile

@@ -107,8 +107,8 @@ RUN ARCH=$(case "${TARGETARCH}" in amd64) echo "x86_64" ;; arm64) echo "aarch64"
     curl -fsSL "https://static.rust-lang.org/rustup/dist/${ARCH}-unknown-linux-gnu/rustup-init" -o /usr/local/bin/rustup-init && \
     chmod +x /usr/local/bin/rustup-init
 
-# Set locale
-RUN sed -i '/en_US.UTF-8/s/^# //g' /etc/locale.gen && locale-gen
+# Set locale — generate common UTF-8 locales (override via LANG/LC_ALL env vars)
+RUN sed -i '/\.UTF-8/s/^# //g' /etc/locale.gen && locale-gen
 ENV LANG=en_US.UTF-8
 ENV LANGUAGE=en_US:en
 ENV LC_ALL=en_US.UTF-8