AP-355 Ensure generaAPted redirect are served as text/html mime type

Why

Explain why these changes are necessary.

We need to ensure that page redirects configured in config/tech-docs.yml are served with the right mime type rather than as binaries and downloaded

Update default mime type in nginx.conf
add a helper script preview-with-nginx.sh to build content, dockerize nginx and run locally on port 80

Do you need a tech writer's support, for example to review your PR? YES

git clone https://github.com/govuk-one-login/tech-docs
git checkout branch
build, dockerize and run ngin in a container locally using./preview-with-nginx.sh
open http://localhost/go-live and sure it redirects correctly

This is a bugfix pr to address an issue with redirects so not required

[x] I have checked if any docs change here also requires updates to other repositories (ADRs / RFCs, README.md, Team Manual, elsewhere in these docs)
[x] Where there is any overlap I have updated or opened a PR for corresponding changes