Remove random password in Dockerfiles

[mgjm]

The Dockerfile contained a random password to unlock the git user account. Otherwise the OpenSSH server would complain about a locked account (#377).

A locked account is marked by a ! at the start off the password hash. An alternative is the password hash *. Both are invalid crypt password hashes and therefore no login via password is possible. The ! has the additional meaning of an locked account, whereas * just means no login. See shadow(5).

Additionally the rootless Dockerfile uses the embedded SSH server. And I think the embedded SSH server does not depend on the git user password at all. Therefore we can leave the password at the default (disabled and locked via !).

ToDo:

• Verify that the ssh access to the (root) docker image still works
• Verify that the ssh access to the rootless docker image still works
  • If the rootless image does not work, revert 04390f5

[mgjm]

I just verified that the login in the main Docker image works with the new * password hash and that the rootless image works with a locked account.

From my side this PR is now ready for review / merge.

[6543]

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