Automatically create admin account

[cjcolvar]

Description

For ease of testing an admin account should be automatically created when the avalon service comes up the first time. This could be done based upon the presence of an environment variable and skipped if that variable is not present. For example, the bitnami postgres image does this: https://github.com/bitnami/containers/blob/main/bitnami/postgresql/README.md#creating-a-database-user-on-first-run

Done Criteria

• Admin account is created upon first run of the Avalon service if none yet exist

Suggested by @zhangyilibecnu