func (s *ContainerRegistryService) DeleteRegistryRepositoryTag(pid interface{}, repository int, tagName string, options ...OptionFunc) (*Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, err
	}
	u := fmt.Sprintf("projects/%s/registry/repositories/%d/tags/%s",
		pathEscape(project),
		repository,
		tagName,
	)
	req, err := s.client.NewRequest("DELETE", u, nil, options)
	if err != nil {
		return nil, err
	}
	return s.client.Do(req, nil)
}
DeleteRegistryRepositoryTag deletes a registry repository tag.

GitLab API docs:
https://docs.gitlab.com/ee/api/container_registry.html#delete-a-repository-tag
package gitlab
import (
	"fmt"
	"time"
)
type ContainerRegistryService struct {
	client *Client
}
type RegistryRepository struct {
	ID        int        `json:"id"`
	Name      string     `json:"name"`
	Path      string     `json:"path"`
	Location  string     `json:"location"`
	CreatedAt *time.Time `json:"created_at"`
}
func (s RegistryRepository) String() string {
	return Stringify(s)
}
type RegistryRepositoryTag struct {
	Name          string     `json:"name"`
	Path          string     `json:"path"`
	Location      string     `json:"location"`
	Revision      string     `json:"revision"`
	ShortRevision string     `json:"short_revision"`
	Digest        string     `json:"digest"`
	CreatedAt     *time.Time `json:"created_at"`
	TotalSize     int        `json:"total_size"`
}
func (s RegistryRepositoryTag) String() string {
	return Stringify(s)
}
type ListRegistryRepositoriesOptions ListOptions
func (s *ContainerRegistryService) ListRegistryRepositories(pid interface{}, opt *ListRegistryRepositoriesOptions, options ...OptionFunc) ([]*RegistryRepository, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/registry/repositories", pathEscape(project))
	req, err := s.client.NewRequest("GET", u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	var repos []*RegistryRepository
	resp, err := s.client.Do(req, &repos)
	if err != nil {
		return nil, resp, err
	}
	return repos, resp, err
}
func (s *ContainerRegistryService) DeleteRegistryRepository(pid interface{}, repository int, options ...OptionFunc) (*Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, err
	}
	u := fmt.Sprintf("projects/%s/registry/repositories/%d", pathEscape(project), repository)
	req, err := s.client.NewRequest("DELETE", u, nil, options)
	if err != nil {
		return nil, err
	}
	return s.client.Do(req, nil)
}
type ListRegistryRepositoryTagsOptions ListOptions
func (s *ContainerRegistryService) ListRegistryRepositoryTags(pid interface{}, repository int, opt *ListRegistryRepositoryTagsOptions, options ...OptionFunc) ([]*RegistryRepositoryTag, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/registry/repositories/%d/tags",
		pathEscape(project),
		repository,
	)
	req, err := s.client.NewRequest("GET", u, opt, options)
	if err != nil {
		return nil, nil, err
	}
	var tags []*RegistryRepositoryTag
	resp, err := s.client.Do(req, &tags)
	if err != nil {
		return nil, resp, err
	}
	return tags, resp, err
}
func (s *ContainerRegistryService) GetRegistryRepositoryTagDetail(pid interface{}, repository int, tagName string, options ...OptionFunc) (*RegistryRepositoryTag, *Response, error) {
	project, err := parseID(pid)
	if err != nil {
		return nil, nil, err
	}
	u := fmt.Sprintf("projects/%s/registry/repositories/%d/tags/%s",
		pathEscape(project),
		repository,
		tagName,
	)
	req, err := s.client.NewRequest("GET", u, nil, options)
	if err != nil {
		return nil, nil, err
	}
	tag := new(RegistryRepositoryTag)
	resp, err := s.client.Do(req, &tag)
	if err != nil {
		return nil, resp, err
	}
	return tag, resp, err
}