For that manual, we assume that you've already got a working Keycloak installation. Otherwise take a look at Install Keycloak as Docker Container with Apache Web Server as Reverse Proxy and a local PostgreSQL first.
The manual was written for Debian Stretch, but should also work with other distributions.

Install openLDAP:

aptitude install slapd ldap-utils

We only want to accept localhost and docker connections, so:
edit /etc/default/slapd

SLAPD_SERVICES="ldap://127.0.0.1 ldap://172.17.0.1 ldapi:///"

restart ldap server

systemctl restart slapd

If you changed the above setting, that slapd should not listen on all hosts, then you also need to tell slapd it should not start up before docker. Otherwise the dockerinterface is not ready to bind to it:

systemctl edit slapd.service

In the editor that was opened enter:

[Unit]
After=docker.service

create file /tmp/test.ldif:

dn: ou=People,dc=example,dc=com
ou: People
objectClass: organizationalUnit

dn: ou=Group,dc=example,dc=com
ou: Group
objectClass: organizationalUnit

execute:

 
ldapadd -x -D cn=admin,dc=example,dc=com -W -f /tmp/test.ldif 

create file member.ldif :

dn: cn=module,cn=config
cn: module
objectclass: olcModuleList
objectclass: top
olcmoduleload: memberof
olcmodulepath: /usr/lib/ldap

dn: olcOverlay={0}memberof,olcDatabase={1}mdb,cn=config
objectClass: olcConfig
objectClass: olcMemberOf
objectClass: olcOverlayConfig
objectClass: top
olcOverlay: memberof 

execute

ldapadd -Y EXTERNAL -Q -H ldapi:/// -f member.ldif

new file refint.ldif

dn: cn=module{1},cn=config
add: olcmoduleload
olcmoduleload: refint

execute

ldapmodify -Q -Y EXTERNAL -H ldapi:/// -f refint.ldif

create file refintconf.ldif

dn: olcOverlay={1}refint,olcDatabase={1}mdb,cn=config
objectClass: olcConfig
objectClass: olcOverlayConfig
objectClass: olcRefintConfig
objectClass: top
olcOverlay: {1}refint
olcRefintAttribute: memberof member manager owner

execute

ldapadd -Q -Y EXTERNAL -H ldapi:/// -f refintconf.ldif

createfile index.ldif

dn: olcDatabase={1}mdb,cn=config
changetype: modify
add: olcDbIndex
olcDbIndex: memberOf, mail eq

execute:

ldapadd -Q -Y EXTERNAL -H ldapi:/// -f index.ldif

create file database.ldif to increase the maxsize of 1GB of the database.I once run out of space after only adding a few 100 users…

dn: olcDatabase={1}mdb,cn=config
changetype: modify
replace: OlcDbMaxSize
OlcDbMaxSize: 20073741824

Execute:

ldapmodify -Q -Y EXTERNAL -H ldapi:/// -f database.ldif

If you didn't create a new realm besides the master realm yet, do that now.

In the new realm click on “User Federation” and then on “Add Provider…”/“Ldap”.

Set following settings:

Then Save everything and click on Mappers.

Create a new mapper with following settings:

Then edit the mapping: “first name” and change:

Create a new mapping

Now newly created and changes to users should be synced to the ldap directory.