document pre-docker build command (#2164)

Co-authored-by: Georg Lauterbach <44545919+georglauterbach@users.noreply.github.com>

docs/content/examples/tutorials/docker-build.md

@@ -0,0 +1,22 @@
+---
+title: 'Tutorials | Docker Build'
+---
+
+## Building your own Docker image
+
+You'll need to retrieve the git submodules prior to building your own Docker image. From within your copy of the git repo run the following to retrieve the submodules and build the Docker image:
+
+```sh
+git submodule update --init --recursive
+docker build .
+```
+
+Or, you can clone and retrieve the submodules in one command:
+```sh
+git clone --recurse-submodules https://github.com/docker-mailserver/docker-mailserver
+```
+
+Retrieving the git submodules will fix the error:
+```
+COPY failed: file not found in build context or excluded by .dockerignore: stat target/docker-configomat/configomat.sh: file does not exist
+```

docs/mkdocs.yml

@@ -146,7 +146,8 @@ nav:
   - 'Examples':
     - 'Tutorials':
       - 'Basic Installation': examples/tutorials/basic-installation.md
-      - 'Mail-Server behind a Proxy': examples/tutorials/mailserver-behind-proxy.md
+      - 'Mailserver behind Proxy': examples/tutorials/mailserver-behind-proxy.md
+      - 'Building your own Docker image': examples/tutorials/docker-build.md
       - 'Blog Posts': examples/tutorials/blog-posts.md
     - 'Use Cases':
       - 'Forward-Only Mail-Server with LDAP': examples/uses-cases/forward-only-mailserver-with-ldap-authentication.md