Restrict `[actions].DEFAULT_ACTIONS_URL` to only `github` or `self` (#25581)

Resolve #24789

## :warning: BREAKING :warning:

Before this, `DEFAULT_ACTIONS_URL` cound be set to any custom URLs like
`https://gitea.com` or `http://your-git-server,https://gitea.com`, and
the default value was `https://gitea.com`.

But now, `DEFAULT_ACTIONS_URL` supports only
`github`(`https://github.com`) or `self`(the root url of current Gitea
instance), and the default value is `github`.

If it has configured with a URL, an error log will be displayed and it
will fallback to `github`.

Actually, what we really want to do is always make it
`https://github.com`, however, this may not be acceptable for some
instances of internal use, so there's extra support for `self`, but no
more, even `https://gitea.com`.

Please note that `uses: https://xxx/yyy/zzz` always works and it does
exactly what it is supposed to do.

Although it's breaking, I belive it should be backported to `v1.20` due
to some security issues.

Follow-up on the runner side:

- https://gitea.com/gitea/act_runner/pulls/262
- https://gitea.com/gitea/act/pulls/70

1 files changed, 11 insertions, 28 deletions

diff --git a/docs/content/doc/administration/config-cheat-sheet.en-us.md b/docs/content/doc/administration/config-cheat-sheet.en-us.md
index 2b95110349..aefa351ecd 100644
--- a/docs/content/doc/administration/config-cheat-sheet.en-us.md
+++ b/docs/content/doc/administration/config-cheat-sheet.en-us.md
@@ -1376,39 +1376,22 @@ PROXY_HOSTS = *.github.com
 ## Actions (`actions`)
 
 - `ENABLED`: **false**: Enable/Disable actions capabilities
-- `DEFAULT_ACTIONS_URL`: **https://gitea.com**: Default address to get action plugins, e.g. the default value means downloading from "<https://gitea.com/actions/checkout>" for "uses: actions/checkout@v3"
+- `DEFAULT_ACTIONS_URL`: **github**: Default platform to get action plugins, `github` for `https://github.com`, `self` for the current Gitea instance.
 - `STORAGE_TYPE`: **local**: Storage type for actions logs, `local` for local disk or `minio` for s3 compatible object storage service, default is `local` or other name defined with `[storage.xxx]`
 - `MINIO_BASE_PATH`: **actions_log/**: Minio base path on the bucket only available when STORAGE_TYPE is `minio`
 
-`DEFAULT_ACTIONS_URL` indicates where should we find the relative path action plugin. i.e. when use an action in a workflow file like
-
-```yaml
-name: versions
-on:
-  push:
-    branches:
-      - main
-      - releases/*
-jobs:
-  build:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v3
-```
-
-Now we need to know how to get actions/checkout, this configuration is the default git server to get it. That means we will get the repository via git clone ${DEFAULT_ACTIONS_URL}/actions/checkout and fetch tag v3.
-
-To help people who don't want to mirror these actions in their git instances, the default value is https://gitea.com
-To help people run actions totally in their network, they can change the value and copy all necessary action repositories into their git server.
+`DEFAULT_ACTIONS_URL` indicates where the Gitea Actions runners should find the actions with relative path.
+For example, `uses: actions/checkout@v3` means `https://github.com/actions/checkout@v3` since the value of `DEFAULT_ACTIONS_URL` is `github`.
+And it can be changed to `self` to make it `root_url_of_your_gitea/actions/checkout@v3`.
 
-Of course we should support the form in future PRs like
-
-```yaml
-steps:
-  - uses: gitea.com/actions/checkout@v3
-```
+Please note that using `self` is not recommended for most cases, as it could make names globally ambiguous.
+Additionally, it requires you to mirror all the actions you need to your Gitea instance, which may not be worth it.
+Therefore, please use `self` only if you understand what you are doing.
 
-although Github don't support this form.
+In earlier versions (<= 1.19), `DEFAULT_ACTIONS_URL` cound be set to any custom URLs like `https://gitea.com` or `http://your-git-server,https://gitea.com`, and the default value was `https://gitea.com`.
+However, later updates removed those options, and now the only options are `github` and `self`, with the default value being `github`.
+However, if you want to use actions from other git server, you can use a complete URL in `uses` field, it's supported by Gitea (but not GitHub).
+Like `uses: https://gitea.com/actions/checkout@v3` or `uses: http://your-git-server/actions/checkout@v3`.
 
 ## Other (`other`)