My image also was missing this dependency for mod_auth_mellon
apt-get install libxmlsec1-openssl
Here are my additions to the config
<If "! %{HTTP_COOKIE} =~ /auth_/ && \
! %{REQUEST_URI} = '/${SITE}/check_mk/register_agent.py' && \
! %{REQUEST_URI} = '/${SITE}/check_mk/webapi.py' && \
! %{REQUEST_URI} = '/${SITE}/check_mk/run_cron.py' && \
! %{REQUEST_URI} = '/${SITE}/check_mk/automation.py' && \
! %{REQUEST_URI} -strmatch '/${SITE}/check_mk/api/*' && \
! %{REQUEST_URI} = '/${SITE}/check_mk/deploy_agent.py' && \
! %{QUERY_STRING} =~ /(_secret=|auth_|register_agent)/ && \
! %{REQUEST_URI} =~ m#^/${SITE}/(omd/|check_mk/(images/.*\.png|login\.py|.*\.(css|js)))# && \
! %{REQUEST_URI} =~ m#^/${SITE}/(omd/|check_mk/(images/.*\.svg|login\.py|.*\.(css|js)))# ">
MellonSecureCookie On
MellonCookieSameSite None