The term gateway is often the most suitable choice, as Don Bron points out. As he notes, the word often (in U.S. usage, anyway) carries the sense of being a way station on a path toward perdition or criminality—though it can be neutral in meaning or even positive. For example, the North American cities of Winnipeg (Manitoba), Tulsa (Oklahoma), Fort Wayne (Indiana), Omaha (Nebraska), Fargo (North Dakota), and St. Louis (Missouri) all claim to be "the Gateway to the West"—a point of civic pride.
But for people on a trajectory from bad life choices to good ones, gateway isn't a great choice because of its association with a downward spiral toward incarceration or institutionalization. For people working toward rehabilitation, a popular alternative designation is halfway house, as this definition from Merriam-Webster's Eleventh Collegiate Dictionary suggests:
halfway house n (1694) ... 2 : a residence for individuals after release from institutionalization(as for mental disorder, drug addiction, or criminal activity) that is designed to facilitate their readjustment to private life
Halfway house is usually intended literally as a house of the type that the Eleventh Collegiate describes, but it can also be used figuratively, as in this example from Philippe de Lombaerde, Antoni Estevadeordal & Kati Suominen, Governing Regional Integration for Development: Monitoring Experiences, Methods and Prospects (2008):
In a nutshell, regionalization is an intermediate position between the backlash of laissez-faire and the unfeasibility of a world government, or, as pointed out by Nye, 'a halfway house between the nation-state and a world that is not willing to become one' [citation omitted].
And this one from Sheri Berman, The Primacy of Politics: Social Democracy and the Making of Europe's Twentieth Century (2006):
Modern scholars, meanwhile, have often failed to appreciate social democracy's ideological distinctiveness. Most work on the subject in recent decades adopts one of two perspectives. The first, often espoused by critics, sees social democracy as an unstable halfway house between Marxism and liberalism, cobbled together from elements of incompatible traditions. In this view, social democrats are socialists without the courage of revolutionary conviction or socialists who have chosen ballots over bullets.