Support for self-managed gitlab instance

[ax1036]

It should be nice to support self managed gitlab, make this hard coded link configurable

vscode-docker/src/tree/registries/gitLab/GitLabAccountTreeItem.ts

Line 23 in aad5061

public baseUrl: string = 'https://gitlab.com/';

[bwateratmsft]

Same as #1832. Since we're tracking all the registry asks in #869 I'll resolve this as dupe.