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>

Dockerfile

@@ -14,6 +14,7 @@ RUN apk --no-cache add \
     s6 \
     curl \
     openssh \
+    gettext \
     tzdata
 RUN addgroup \
     -S -g 1000 \