Add GRAV_SUBDIR argument

GRAV_SUBDIR argument should never start with a forward slash. Defaults to 'html' (without quotes).

Dockerfile

@@ -53,12 +53,14 @@ USER www-data
 
 # Define Grav specific version of Grav or use latest stable
 ARG GRAV_VERSION=latest
+ARG GRAV_SUBDIR=html
 
 # Install grav
 WORKDIR /var/www
 RUN curl -o grav-admin.zip -SL https://getgrav.org/download/core/grav-admin/${GRAV_VERSION} && \
     unzip grav-admin.zip && \
-    mv -T /var/www/grav-admin /var/www/html && \
+    mkdir -p ${GRAV_SUBDIR%/*}
+    mv -T /var/www/grav-admin "var/www/$GRAV_SUBDIR" && \
     rm grav-admin.zip
 
 # Create cron job for Grav maintenance scripts