feat: Provide environment variable to customize varnishd startup parameters and increase limits, fixes #41

[das-peter]

The Issue

• Allow to set custom varnish daemon arguments via env-var #41

It would be nice to be able to easily adjust the varnish startup parameters.
Env-Variables are a bit more intuitive than docker compose file creation.
Further I think it would be beneficial to increase the default varnishd limits for http headers and payload sizes.
These payloads often are larger on development environments and can cause hard to track issues.

How This PR Solves The Issue

Adds a env-variable for the parameters and defines larger limits by defaiult.

Manual Testing Instructions

ddev add-on get https://github.com/CandoImage/ddev-varnish/tarball/20250721_das-peter_41_provide-env-var-to-set-varnishd-parameters
ddev dotenv set .ddev/.env.varnish --varnish-varnishd-params='-p http_max_hdr=12345'
ddev restart
ddev varnishadm param.show http_max_hdr

Automated Testing Overview

Added new test to check if customization is picked up:
bats ./tests/test.bats --filter "customize varnishd startup parameters"

Release/Deployment Notes

[rfay]

So awesome that you provided test and testing instructions!

[das-peter]

So awesome that you provided test and testing instructions!

We'll I seem to struggle with the tests ;) But will mark this ready once all is passing.
And hey, another testing tool under my belt - didn't know bats yet.

[rfay]

Bats is supposed to be easy but it isn't always easy :)

[rfay]

Yay, congrats!

[stasadev]

Thank you!

Manual testing works well, my suggestion is to add more checks to test for each of these values in all cases.

[stasadev]

Looks good to me, thank you!