added user-requirements.txt install workflow

Dockerfile

@@ -34,7 +34,7 @@ WORKDIR /tmp
 COPY material material
 COPY package.json package.json
 COPY README.md README.md
-COPY requirements.txt requirements.txt
+COPY *requirements.txt ./
 COPY pyproject.toml pyproject.toml
 
 # Perform build and cleanup artifacts and caches
@@ -66,6 +66,10 @@ RUN \
       "pillow>=9.0" \
       "cairosvg>=2.5"; \
   fi \
+&& \
+  if [ -e user-requirements.txt ]; then \
+    pip install -U -r user-requirements.txt; \
+  fi \
 && \
   apk del .build \
 && \

docs/getting-started.md

@@ -117,11 +117,12 @@ The following plugins are bundled with the Docker image:
 
     Material for MkDocs only bundles selected plugins in order to keep the size
     of the official image small. If the plugin you want to use is not included, 
-    create a new `Dockerfile` and extend the official Docker image:
+    create a `user-requirements.txt` file in the repository root with the packages
+    you want to install additionally, e.g.:
 
-    ``` Dockerfile
-    FROM squidfunk/mkdocs-material
-    RUN pip install ...
+    ``` txt title="user-requirements.txt"
+    mkdocs-macros-plugin==0.7.0
+    mkdocs-glightbox>=0.3.1
     ```
 
     Next, you can build the image with the following command:
@@ -130,7 +131,8 @@ The following plugins are bundled with the Docker image:
     docker build -t squidfunk/mkdocs-material .
     ```
 
-    The new image can be used exactly like the official image.
+    The new image will have additional packages installed and can be used
+    exactly like the official image.
 
 ### with git
 

docs/insiders/getting-started.md

@@ -79,6 +79,9 @@ docker login -u ${GH_USERNAME} -p ${GHCR_TOKEN} ghcr.io
 docker pull ghcr.io/${GH_USERNAME}/mkdocs-material-insiders
 ```
 
+Should you wish to add additional plugins to the insiders container image, follow the steps
+outlined in the [Getting Started guide](../getting-started.md#with-docker).
+
   [^2]:
     Earlier, Insiders provided a dedicated Docker image which was available to
     all sponsors. On March 21, 2021, the image was deprecated for the reasons