(core) give instructions on using Grist with docker

Summary: cleans up docker build and instructions.

Test Plan: docker image and instructions tested manually

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2648

app/server/lib/log.ts

@@ -64,7 +64,7 @@ function timestamp() {
 
 const fileTransportOptions = {
   stream: process.stderr,
-  level: 'debug',
+  level: process.env.GRIST_LOG_LEVEL || 'debug',
   timestamp: log.timestamp,
   colorize: true,
   json: process.env.GRIST_HOSTED_VERSION ? true : false