contrib: remove jinja2 extension Dockerfile example, inline in README instead

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

contrib/docker/cookiecutter-with-jinja2-extensions/Dockerfile

@@ -1,11 +0,0 @@
-FROM alpine:3.16
-
-RUN apk add --update \
-	git \
-	python \
-	python-dev \
-	py-pip \
-	g++ && \
-	pip install cookiecutter jinja2_custom_filters_extension && \
-	apk del g++ py-pip python-dev && \
-	rm -rf /var/cache/apk/*

contrib/docker/cookiecutter-with-jinja2-extensions/README.md

@@ -51,7 +51,7 @@ steps:
       imageName: 'foo/custom-built-cookiecutter-image-with-extensions'
 ```
 
-See for example, the [`Dockerfile`](./Dockerfile) in this directory.
+For example, you can `pip install jinja2_custom_filters_extension` as part of your cookiecutter Dockerfile.
 
 ### Instructing Cookiecutter to use the extension