Allow env var to override Pelican config file

It is common to use pelicanconf.py for local development and then
publishconf.py for production - allowing this to be configurable
lets people manage multiple Pelican configs as required.

README.md

@@ -15,6 +15,7 @@ Please ensure a `requirements.txt` is present for your site and installs
 ## Environment variables
 
   - `GH_PAGES_BRANCH` (optional): override the default `gh-pages` deployment branch
+  - `PELICAN_CONFIG_FILE` (optional): override the default `pelicanconf.py` config file
 
 ## History
 

entrypoint.sh

@@ -13,7 +13,7 @@ ssh-keyscan -t rsa github.com > /root/.ssh/known_hosts && \
 echo "${GIT_DEPLOY_KEY}" > /root/.ssh/id_rsa && \
 chmod 400 /root/.ssh/id_rsa
 echo '=================== Build site ==================='
-pelican content -o output -s pelicanconf.py
+pelican content -o output -s ${PELICAN_CONFIG_FILE:=pelicanconf.py}
 echo '=================== Publish to GitHub Pages ==================='
 cd output
 # shellcheck disable=SC2012