Pode ser especificado incorretamente o endereço IP do docker. Aqui está o meu web.config:
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<rewrite>
<rules>
<clear />
<rule name="Let's encypt" stopProcessing="true">
<match url="^\.well-known.*$" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="false" />
<action type="None" />
</rule>
<rule name="gitlab forwarding" stopProcessing="true">
<match url="(.*)" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="false" />
<serverVariables>
<set name="HTTP_X_FORWARDED_HOST" value="your.public.domain" />
</serverVariables>
<action type="Rewrite" url="http://192.168.99.100:10080/{R:1}" />
</rule>
</rules>
</rewrite>
</system.webServer>
</configuration>