Limit locales to 16 common languages, document how to add more

Reduces locale generation from 200+ to 16 targeted locales (major world
languages + Nordic + key European). Saves build time and image size.
Users can add more at runtime via locale-gen.

Dockerfile

@@ -110,7 +110,8 @@ RUN ARCH=$(case "${TARGETARCH}" in amd64) echo "x86_64" ;; arm64) echo "aarch64"
     chmod +x /usr/local/bin/rustup-init
 
 # 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
+# To add more locales, run: sudo sed -i '/<locale>.UTF-8/s/^# //g' /etc/locale.gen && sudo locale-gen
+RUN sed -i -E '/(en_US|en_GB|sv_SE|da_DK|nb_NO|fi_FI|de_DE|fr_FR|es_ES|it_IT|pt_BR|nl_NL|pl_PL|ja_JP|ko_KR|zh_CN)\.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