Support running under UID 500

When started by the Flatcar core user, the SDK failed to use UID 500
because inside the SDK there already is the core user from nss-altfiles
with the same ID. This way, the SDK user was continuing with UID 1000
and had permission errors.
Allow to reuse an existing ID for the SDK user. However, this only
works when usermod doesn't find a process that uses this ID, and we had
a race between the SDK entry points called by "docker start" and by
"docker exec". The race is unwanted anyway because we don't want to
execute the commands while setup_board is still running. Solve it by
setting the entrypoint for "docker start" directly to "bash -l" in
"docker create" (this is also what the entry point does as last step:
sudo su -l).

run_sdk_container

@@ -129,7 +129,8 @@ if [ -z "$stat" ] ; then
        -e SDK_GROUP_ID="$(id -g)" \
        --name="$name" \
        --hostname="$hostname" \
-       "${container_image_name}"
+       --entrypoint /bin/bash \
+       "${container_image_name}" -l
 fi
 
 if [ "$stat" != "Up" ] ; then

sdk_lib/sdk_entry.sh

@@ -1,10 +1,11 @@
 #!/bin/bash
 
 if [ -n "${SDK_USER_ID:-}" ] ; then
-    usermod -u $SDK_USER_ID sdk
+    # If the "core" user from /usr/share/baselayout/passwd has the same ID, allow to take it instead
+    usermod --non-unique -u $SDK_USER_ID sdk
 fi
 if [ -n "${SDK_GROUP_ID:-}" ] ; then
-    groupmod -g $SDK_GROUP_ID sdk
+    groupmod --non-unique -g $SDK_GROUP_ID sdk
 fi
 
 chown -R sdk:sdk /home/sdk