Rework containers to create the current user dynamically instead of using canned /etc/passwd and /etc/group

[rfay]

In our containers we use a canned /etc/passwd and /etc/group, just so every user (but only below id 60000) can have a name and be looked up.

We'd be a lot better to actually create the user user useradd or the like on login, then use the actual id. That would allow us to use high windows and mac uids without strain. And it would leave room in the container for when people add a package that creates a user.

So:

• Use default /etc/passwd and /etc/group
• Dynamically create user on container startup, with same name and id and primary group and the host user.

[rfay]

I took a look at this today and was unable to use either sudo or su to create an arbitrary user when working from a user with no name.

Options:

• Use "build" to build in the right user. This would be lovely, but would mean that build was always used.
• Add an extra stage where a root exec happened to set up the user.

[rfay]

Fixed in #1688