chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/healthcheck/handler.go

@@ -12,11 +12,9 @@ type handler struct {
 	logger      Logger
 }
 
-var errHealthcheckNotRunYet = errors.New("healthcheck did not run yet")
-
 func newHandler(logger Logger) *handler {
 	return &handler{
-		healthErr: errHealthcheckNotRunYet,
+		healthErr: errors.New("healthcheck did not run yet"),
 		logger:    logger,
 	}
 }