build: exclude cloud auth plugins from container image

[AkashKumar7902]

Fixes #5085

What this PR does

• limits the standard container build tags to osusergo and netgo
• removes the gateway provisioner's unconditional GCP authentication plugin import
• retains the tagged GCP and OIDC plugins for normal local Makefile builds

Why

The Dockerfile began using make build during an earlier refactor and consequently inherited the Makefile's local-development gcp and oidc tags. The gateway provisioner also imported the GCP plugin unconditionally, bypassing the existing gcp build constraint.

Impact

The standard in-cluster Contour image no longer links the GCP or OIDC client authentication plugins. Local make build and make install builds continue to include both plugins.

Testing

• compared dependency graphs under osusergo netgo and oidc gcp osusergo netgo
• built and tested the Contour binary with both tag sets
• ran the full Go test suite with both tag sets
• ran go vet with both tag sets
• built and ran the standard scratch container image, then inspected its binary for auth-plugin symbols
• ran make lint-codespell, make lint-flags, and git diff --check

Release note

Requested label: release-note/small. The corresponding numbered changelog is included.

[AkashKumar7902]

Could a maintainer please apply the release-note/small label? I do not have permission to label the upstream PR. The matching numbered changelog is already included.

[github-actions]

The Contour project currently lacks enough contributors to adequately respond to all PRs.

This bot triages PRs according to the following rules:

• After 30d of inactivity, lifecycle/stale is applied
• After 60d of inactivity since lifecycle/stale was applied, the PR is closed

You can:

• Ensure your PR is passing all CI checks. PRs that are fully green are more likely to be reviewed. If you are having trouble with CI checks, reach out to the #contour channel in the Kubernetes Slack workspace.
• Mark this PR as fresh by commenting or pushing a commit
• Close this PR
• Offer to help out with triage

Please send feedback to the #contour channel in the Kubernetes Slack