Can't reach shell when running

**Actual Behaviour**

I am unable to reach the shell at http://localhost:8000/shell. I get
Dynamodb Local Started, Visit: http://localhost:8000/shell
Serverless: DynamoDB - created table TestTable-dev

when I run sls dynamodb start --migrate

The process seems to exit after that prints out

**Expected Behaviour**

I should be able to access the shell and the process shouldn't exit

**Steps to reproduce it**

sls dynamodb install

sls dynamodb start --migrate

Reproduce the issue with `sls dynamodb install` followed by `sls dynamodb start --migrate`, then inspect the command's startup and migration flow. Done means the process remains running after creating `TestTable-dev` and `http://localhost:8000/shell` is accessible.

Written by the indexing model from the issue text.