Support for custom Python dependencies

* If the user has provided a `requirements.txt` file in the root
  directory of the sources it will be installed with `pip`.

Dockerfile

@@ -27,8 +27,8 @@ USER root
 # as well as the specified version of MkDocs and the Material theme.
 RUN yum install -y rh-python36-python-pip && \
     yum clean all && \
-    /opt/rh/rh-python36/root/usr/bin/pip install --upgrade pip && \
-    /opt/rh/rh-python36/root/usr/bin/pip install \
+    /opt/rh/rh-python36/root/usr/bin/pip install --cache-dir /root --upgrade pip && \
+    /opt/rh/rh-python36/root/usr/bin/pip install --cache-dir /root \
         mkdocs==${MKDOCS_VERSION} \
         mkdocs-material==${MATERIAL_VERSION}
 

assemble

@@ -6,17 +6,27 @@ set -e
 export LC_ALL=en_US.utf-8
 export LANG=en_US.utf-8
 
-echo "---> Building the MkDocs static site"
-
 # Save the current directory.
 cwd=$(pwd)
 
 # Move the original sources to another directory.
 mv /tmp/src /tmp/src-original
 
-# Build the MkDocs static site in the standard directory.
+# Go to the directory where the original sources have been moved to.
 cd /tmp/src-original/
-/opt/rh/rh-python36/root/usr/bin/mkdocs build --clean --site-dir /tmp/src/
+
+# Install custom Python dependencies.
+if [ -f ./requirements.txt ]; then
+  echo "---> Installing custom Python dependencies..."
+  export PYTHONPATH=$PYTHONPATH:/opt/app-root/lib/python3.6/site-packages
+  /opt/rh/rh-python36/root/bin/pip install \
+    --cache-dir /opt/app-root/.cache \
+    --target /opt/app-root/lib/python3.6/site-packages -r requirements.txt
+fi
+
+# Build the MkDocs static site in the standard directory.
+echo "---> Building the MkDocs static site..."
+/opt/rh/rh-python36/root/bin/mkdocs build --clean --site-dir /tmp/src/
 
 # Move the Nginx-related files back to the standard directory.
 if [ -f ./nginx.conf ]; then
@@ -47,7 +57,7 @@ fi
 # Go back to the current directory.
 cd $cwd
 
-echo "---> Calling the original assemble script"
+echo "---> Calling the original assemble script..."
 
 # Call the original assemble script.
 /usr/libexec/s2i/assemble-original