Allow configuring the buf remote (#121)

Add an input to the setup action that can control which Buf Schema
Registry to login to, which will be useful for users who have a private
BSR instance. I used `domain` as this is what we refer to in the cli
help:
```
$ buf registry login --help
Log in to the Buf Schema Registry

This prompts for your BSR username and a BSR token and updates your .netrc file with these credentials.
The <domain> argument will default to buf.build if not specified.

Usage:
  buf registry login <domain> [flags]
```

Author: rhbuf

README.md

@@ -36,6 +36,7 @@ You can configure `buf-setup-action` with these parameters:
 | `github_token` | The GitHub token to use when making API requests   |                    |
 | `buf_user`     | The username to use for logging into Buf Schema registry.                                               |                    |
 | `buf_api_token` | The API token to use for logging into Buf Schema registry.                                                                                                            |                    |
+| `buf_domain`    | The domain of the Buf Schema Registry to login to.     | buf.build |
 
 > These parameters are derived from [`action.yml`](./action.yml). <br>
 #### Version
@@ -117,6 +118,12 @@ Note that this only authenticate you with the `buf` cli. You cannot access your
 packages in BSR. If you need to access your private remote packages, supply the username and Buf
 API Token [as parameters](#buf-username-and-buf-api-token). 
 
+#### Buf domain
+
+If you are working with a private BSR then you can set the `buf_domain` input to the domain of
+your instance. Please ensure that you are using a token created on your instance (e.g. `https://buf.example.com/settings/user`) and not from the public BSR at `https://buf.build`.
+
+
 #### Installing `protoc`
 
 In most cases, you _don't_ need to install [`protoc`][protoc] for Buf's GitHub Actions, but some

action.yml

@@ -18,6 +18,10 @@ inputs:
   buf_api_token:
     description: The API token to use for logging into Buf Schema registry.
     required: false
+  buf_domain:
+    description: The domain of the Buf Schema Registry to login to.
+    required: false
+    default: 'buf.build'
 runs:
   using: "node16"
   main: "./dist/main.js"

dist/main.js

@@ -79,14 +79,15 @@ async function runSetup(): Promise<null | Error> {
   core.info(`Successfully setup buf version ${version}`);
   core.info(cp.execSync(`${binaryPath} --version`).toString());
 
+  const bufDomain = core.getInput("buf_domain");
   const bufUser = core.getInput("buf_user");
   const bufAPIToken = core.getInput("buf_api_token");
   if (bufUser !== "" && bufAPIToken !== "") {
     core.info(`buf_user and buf_token supplied, logging in...`);
     core.info(
       cp
         .execSync(
-          `${binaryPath} registry login --username ${bufUser} --token-stdin`,
+          `${binaryPath} registry login ${bufDomain} --username ${bufUser} --token-stdin`,
           { input: bufAPIToken }
         )
         .toString()