Add environment variable support for Docker image (#2201)

* Add `gettext` dependencies as we need `envsubst` command;
* Modified s6's gitea setup script, instead of `cp` the template if no
`app.ini` exist, it will substitude the envvars and generate the new
`app.ini`;
* Make `/docker/etc/templates/app.ini` a template contains environment
variables;

Signed-off-by: Tao Wang <twang2218@gmail.com>

docker/etc/s6/gitea/setup

@@ -12,7 +12,29 @@ fi
 
 if [ ! -f /data/gitea/conf/app.ini ]; then
     mkdir -p /data/gitea/conf
-    cp /etc/templates/app.ini /data/gitea/conf/app.ini
+
+    # Set INSTALL_LOCK to true only if SECRET_KEY is not empty and
+    # INSTALL_LOCK is empty
+    if [ -n "$SECRET_KEY" ] && [ -z "$INSTALL_LOCK" ]; then
+        INSTALL_LOCK=true
+    fi
+
+    # Substitude the environment variables in the template
+    APP_NAME=${APP_NAME:-"Gitea: Git with a cup of tea"} \
+    APP_MODE=${APP_MODE:-"dev"} \
+    SSH_DOMAIN=${SSH_DOMAIN:-"localhost"} \
+    HTTP_PORT=${HTTP_PORT:-"3000"} \
+    ROOT_URL=${ROOT_URL:-""} \
+    DISABLE_SSH=${DISABLE_SSH:-"false"} \
+    SSH_PORT=${SSH_PORT:-"22"} \
+    DB_TYPE=${DB_TYPE:-"sqlite3"} \
+    DB_HOST=${DB_HOST:-"localhost:3306"} \
+    DB_NAME=${DB_NAME:-"gitea"} \
+    DB_USER=${DB_USER:-"root"} \
+    DB_PASSWD=${DB_PASSWD:-""} \
+    INSTALL_LOCK=${INSTALL_LOCK:-"false"} \
+    SECRET_KEY=${SECRET_KEY:-""} \
+    envsubst < /etc/templates/app.ini > /data/gitea/conf/app.ini
 fi
 
 chown -R git:git /data/gitea /app/gitea /data/git