Allow Docker image to generate servers.json from environment

In the Docker image, generate the servers.json with a single server
by reading environment variables.

config/docker/servers.json_from_env.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+set -e
+
+ME=$(basename $0)
+
+setup_single_shlink_server() {
+  [ -n "$SHLINK_CLIENT_SERVER_URL" ] || return 0
+  [ -n "$SHLINK_CLIENT_API_KEY" ] || return 0
+  local name="${SHLINK_CLIENT_SERVER_NAME:-Shlink}"
+  echo "[{\"name\":\"${name}\",\"url\":\"${SHLINK_CLIENT_SERVER_URL}\",\"apiKey\":\"${SHLINK_CLIENT_API_KEY}\"}]" > /usr/share/nginx/html/servers.json
+}
+
+setup_single_shlink_server
+
+exit 0