feat(auth): add auth provider for Bitbucket Server

Signed-off-by: Katharina Sick <katharina.sick@dynatrace.com>

docs/auth/bitbucket/provider.md

@@ -61,7 +61,7 @@ how this is done. Note that for the Bitbucket provider, you'll want to use
 factory.
 
 The `@backstage/plugin-auth-backend` plugin also comes with two built-in
-resolves that can be used if desired. The first one is the
+resolvers that can be used if desired. The first one is the
 `bitbucketUsernameSignInResolver`, which identifies users by matching their
 Bitbucket username to `bitbucket.org/username` annotations of `User` entities in
 the catalog. Note that you must populate your catalog with matching entities or

docs/auth/bitbucketServer/provider.md

@@ -0,0 +1,52 @@
+---
+id: provider
+title: Bitbucket Server Authentication Provider
+sidebar_label: Bitbucket Server
+description: Adding Bitbucket Server OAuth as an authentication provider in Backstage
+---
+
+The Backstage `core-plugin-api` package comes with a Bitbucket Server authentication provider that can authenticate
+users using Bitbucket Server. This does **NOT** work with Bitbucket Cloud.
+
+## Create an Application Link in Bitbucket Server
+
+To add Bitbucket Server authentication, you must create an outgoing application link. Follow the steps described in
+the [Bitbucket Server documentation](https://confluence.atlassian.com/bitbucketserver/configure-an-outgoing-link-1108483656.html)
+to create one.
+
+## Configuration
+
+The provider configuration can then be added to your `app-config.yaml` under the root `auth` configuration:
+
+```yaml
+auth:
+  environment: development
+  providers:
+    bitbucketServer:
+      development:
+        host: bitbucket.org
+        clientId: ${AUTH_BITBUCKET_SERVER_CLIENT_ID}
+        clientSecret: ${AUTH_BITBUCKET_SERVER_CLIENT_SECRET}
+```
+
+The Bitbucket Server provider is a structure with two configuration keys:
+
+- `clientId`: The client ID that was generated by Bitbucket, e.g. `b0f868455c15dcdff5c5fb5d173ae684`.
+- `clientSecret`: The client secret tied to the generated client ID.
+
+## Adding the provider to the Backstage frontend
+
+To add the provider to the frontend, add the `bitbucketServerAuthApi` reference and `SignInPage` component as shown
+in [Adding the provider to the sign-in page](../index.md#adding-the-provider-to-the-sign-in-page).
+
+## Using Bitbucket Server for sign-in
+
+In order to use the Bitbucket Server provider for sign-in, you must configure it with a `signIn.resolver`. See
+the [Sign-In Resolver documentation](../identity-resolver.md) for more details on how this is done. Note that for the
+Bitbucket Server provider, you'll want to use `bitbucketServer` as the provider ID,
+and `providers.bitbucketServer.create` for the provider factory.
+
+The `@backstage/plugin-auth-backend` plugin also comes with a built-in resolver that can be used if desired.
+The `emailMatchingUserEntityProfileEmail` identifies users by matching their Bitbucket Server email address to the email
+address of `User` entities in the catalog. Note that you must populate your catalog with matching entities or users will
+not be able to sign in with this resolver.

docs/auth/index.md

@@ -18,6 +18,7 @@ Backstage comes with many common authentication providers in the core library:
 - [Auth0](auth0/provider.md)
 - [Azure](microsoft/provider.md)
 - [Bitbucket](bitbucket/provider.md)
+- [Bitbucket Server](bitbucketServer/provider.md)
 - [Cloudflare Access](cloudflare/access.md)
 - [GitHub](github/provider.md)
 - [GitLab](gitlab/provider.md)