Added SAML2 support

SAML2 is an alternative authorization flow often used in the same places LDAP is.

The documentation for how SAML2 is used can be found here:
https://www.bookstackapp.com/docs/admin/saml2-auth/

docker-entrypoint.sh

@@ -70,6 +70,26 @@ if [ ! -f ".env" ]; then
       LDAP_USER_FILTER=${LDAP_USER_FILTER:-false}
       LDAP_VERSION=${LDAP_VERSION:-false}
 
+      # SAML2 Settings; AUTH_MODE=saml2
+      AUTH_AUTO_INITIATE=${AUTH_AUTO_INITIATE:-false}
+      SAML2_NAME=${SAML2_NAME:-null}
+      SAML2_EMAIL_ATTRIBUTE=${SAML2_EMAIL_ATTRIBUTE:-null}
+      SAML2_EXTERNAL_ID_ATTRIBUTE=${SAML2_EXTERNAL_ID_ATTRIBUTE:-null}
+      SAML2_USER_TO_GROUPS=${SAML2_USER_TO_GROUPS:-false}
+      SAML2_GROUP_ATTRIBUTE=${SAML2_GROUP_ATTRIBUTE:-null}
+      SAML2_DISPLAY_NAME_ATTRIBUTES=${SAML2_DISPLAY_NAME_ATTRIBUTES:-null}
+      SAML2_IDP_ENTITYID=${SAML2_IDP_ENTITYID:-null}
+      SAML2_AUTOLOAD_METADATA=${SAML2_AUTOLOAD_METADATA:-false}
+      SAML2_IDP_SSO=${SAML2_IDP_SSO:-null}
+      SAML2_IDP_SLO=${SAML2_IDP_SLO:-null}
+      SAML2_IDP_x509=${SAML2_IDP_x509:-null}
+      SAML2_IDP_AUTHNCONTEXT=${SAML2_IDP_AUTHNCONTEXT:-false}
+      SAML2_SP_x509=${SAML2_SP_x509:-null}
+      SAML2_SP_x509_KEY=${SAML2_SP_x509_KEY:-null}
+      SAML2_DUMP_USER_DETAILS=${SAML2_DUMP_USER_DETAILS:-false}
+      SAML2_ONELOGIN_OVERRIDES=${SAML2_ONELOGIN_OVERRIDES:-null}
+      SAML2_REMOVE_FROM_GROUPS=${SAML2_REMOVE_FROM_GROUPS:-false}
+
       # Mail settings
       MAIL_DRIVER=${MAIL_DRIVER:-smtp}
       MAIL_HOST=${MAIL_HOST:-localhost}