Solve a coding problem that involves data structures or algorithms relevant to DevOps tasks.
Implement has_dependency_cycle(dependencies) to determine whether a directed dependency graph contains a cycle. Each key is a service or deployment component, and its value lists components that must be deployed before it. Return True if any cycle exists, otherwise return False.
The input is a dictionary of strings to lists of strings. Components mentioned only as dependencies are valid vertices. Use the function signature def has_dependency_cycle(dependencies):.
def has_dependency_cycle(dependencies):