azure: docker: avoid re-creating libgit2 home directory

The Docker entrypoint currently creates the libgit2 user with "useradd
--create-home". As we start the Docker container with two volumes
pointing into "/home/libgit2/", the home directory will already exist.
While useradd(1) copes with this just fine, it will print error messages
to stderr which end up as failures in our Azure pipelines.

Fix this by simply removing the "--create-home" parameter.

azure-pipelines/docker/entrypoint.sh

-#!/bin/bash
-useradd --shell /bin/bash --create-home libgit2
+#!/bin/bash -e
+useradd --shell /bin/bash libgit2
 chown -R $(id -u libgit2) /home/libgit2
 exec gosu libgit2 "$@"