Fix docker user permission issues by creating the user manually (#1183)

DiscordChatExporter.Cli.dockerfile

@@ -46,10 +46,10 @@ ENV DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=false
 ENV LC_ALL=en_US.UTF-8
 ENV LANG=en_US.UTF-8
 
-# Use a non-root user to ensure that the files shared with the host are accessible by the host user.
-# We can use the default user provided by the base image for this purpose.
+# Use a non-root user to ensure that the files shared with the host are accessible by the host user
 # https://github.com/Tyrrrz/DiscordChatExporter/issues/851
-USER app
+RUN adduser --disabled-password --no-create-home dce
+USER dce
 
 # This directory is exposed to the user for mounting purposes, so it's important that it always
 # stays the same for backwards compatibility.