enhance(runner): add extra HTTP headers (#1269)

Add `runner.extra_headers` for reverse proxies that require request headers on the runner's API requests to Gitea, like registration, polling and log and status reports.

Headers the runner sets itself (authentication, User-Agent, protocol headers) stay authoritative. Extra headers are dropped when a redirect changes the hostname or leaves HTTPS, while the runner's own authentication headers follow redirects as before. Invalid or case-insensitively duplicated header names are rejected at config load.

Fixes: https://gitea.com/gitea/runner/issues/171
Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1269
Co-authored-by: bircni <[email protected]>

internal/pkg/config/config.example.yaml

@@ -83,6 +83,11 @@ runner:
   # and github_mirror is not empty. In this case,
   # it replaces https://github.com with the value here, which is useful for some special network environments.
   #github_mirror: ''
+  # Extra HTTP headers for runner API requests to Gitea, e.g. reverse proxy authentication.
+  # Not used for action clones, artifacts, job summaries or jobs. Runner headers take precedence.
+  # Dropped on redirects to another hostname, or to HTTP if the instance URL uses HTTPS.
+  #extra_headers:
+  #  X-Proxy-Token: 'example'
   # When true (the default), fetch only the requested ref of an action repository (e.g. actions/checkout@v4) at depth 1 instead of cloning every branch's full history.
   # Set to false to clone the full history.
   #action_shallow_clone: true