Use CI_API_V4_URL in GitLab if it is available

docs/usage/gitlab.html.md

@@ -7,11 +7,14 @@ blurb: An overview of using Danger with GitLab, and some examples
 ---
 
 To use Danger JS with GitLab: you'll need to create a new account for Danger to use, then set the following environment
-variables on your CI system:
+variable on your CI system:
 
-- `DANGER_GITLAB_HOST` = Defaults to `https://gitlab.com` but you can use it for your own url
 - `DANGER_GITLAB_API_TOKEN` = An access token for the account which will post comments
 
+If you are using a GitLab version prior to 11.7 you will also need to define the following environment variable:
+
+- `DANGER_GITLAB_HOST` = Defaults to `https://gitlab.com` but you can use it for your own url
+
 Then in your Dangerfiles you will have a fully fleshed out `danger.gitlab` object to work with. For example:
 
 ```ts

source/platforms/gitlab/GitLabAPI.ts

@@ -25,11 +25,23 @@ export interface GitLabAPICredentials {
 export function getGitLabAPICredentialsFromEnv(env: Env): GitLabAPICredentials {
   let host = "https://gitlab.com"
   const envHost = env["DANGER_GITLAB_HOST"]
+  const envCIAPI = env["CI_API_V4_URL"]
+
   if (envHost) {
     // We used to support DANGER_GITLAB_HOST being just the host e.g. "gitlab.com"
     // however it is possible to have a custom host without SSL, ensure we only add the protocol if one is not provided
     const protocolRegex = /^https?:\/\//i
     host = protocolRegex.test(envHost) ? envHost : `https://${envHost}`
+  } else if (envCIAPI) {
+    // GitLab >= v11.7 supplies the API Endpoint in an environment variable and we can work out our host value from that.
+    // See https://docs.gitlab.com/ce/ci/variables/predefined_variables.html
+    const hostRegex = /^(https?):\/\/([^\/]+)\//i
+    if (hostRegex.test(envCIAPI)) {
+      const matches = hostRegex.exec(envCIAPI)!
+      const matchProto = matches[1]
+      const matchHost = matches[2]
+      host = `${matchProto}://${matchHost}`
+    }
   }
 
   return {

source/platforms/gitlab/_tests/_gitlab_api.test.ts

@@ -35,6 +35,18 @@ describe("GitLab API", () => {
     )
   })
 
+  it("configures host from CI_API_V4_URL", () => {
+    api = new GitLabAPI(
+      { pullRequestID: "27117", repoSlug: "gitlab-org/gitlab-ce" },
+      getGitLabAPICredentialsFromEnv({
+        CI_API_V4_URL: "https://testciapiv4url.com/api/v4",
+        DANGER_GITLAB_API_TOKEN: "FAKE_DANGER_GITLAB_API_TOKEN",
+      })
+    )
+
+    expect(api.projectURL).toBe("https://testciapiv4url.com/gitlab-org/gitlab-ce")
+  })
+
   it("projectURL is defined", () => {
     expect(api.projectURL).toBe("https://gitlab.com/gitlab-org/gitlab-ce")
   })