Make environment-to-ini support loading key value from file (#24832)

Replace #19857

Close #19856
Close #10311
Close #10123

Major changes:

1. Move a lot of code from `environment-to-ini.go` to `config_env.go` to
make them testable.
2. Add `__FILE` support
3. Update documents
4. Add tests

2 files changed, 24 insertions, 6 deletions

diff --git a/docs/content/doc/installation/with-docker-rootless.en-us.md b/docs/content/doc/installation/with-docker-rootless.en-us.md
index e1073a1d67..b8c76438f9 100644
--- a/docs/content/doc/installation/with-docker-rootless.en-us.md
+++ b/docs/content/doc/installation/with-docker-rootless.en-us.md
@@ -286,9 +286,18 @@ docker-compose up -d
 
 ## Managing Deployments With Environment Variables
 
-In addition to the environment variables above, any settings in `app.ini` can be set or overridden with an environment variable of the form: `GITEA__SECTION_NAME__KEY_NAME`. These settings are applied each time the docker container starts. Full information [here](https://github.com/go-gitea/gitea/tree/main/contrib/environment-to-ini).
-
-These environment variables can be passed to the docker container in `docker-compose.yml`. The following example will enable an smtp mail server if the required env variables `GITEA__mailer__FROM`, `GITEA__mailer__HOST`, `GITEA__mailer__PASSWD` are set on the host or in a `.env` file in the same directory as `docker-compose.yml`:
+In addition to the environment variables above, any settings in `app.ini` can be set
+or overridden with an environment variable of the form: `GITEA__SECTION_NAME__KEY_NAME`.
+These settings are applied each time the docker container starts.
+Full information [here](https://github.com/go-gitea/gitea/tree/main/contrib/environment-to-ini).
+
+These environment variables can be passed to the docker container in `docker-compose.yml`.
+The following example will enable a smtp mail server if the required env variables
+`GITEA__mailer__FROM`, `GITEA__mailer__HOST`, `GITEA__mailer__PASSWD` are set on the host
+or in a `.env` file in the same directory as `docker-compose.yml`.
+
+The settings can be also set or overridden with the content of a file by defining an environment variable of the form:
+`GITEA__section_name__KEY_NAME__FILE` that points to a file.
 
 ```bash
 ...
diff --git a/docs/content/doc/installation/with-docker.en-us.md b/docs/content/doc/installation/with-docker.en-us.md
index 043a10c08f..e70a6ab133 100644
--- a/docs/content/doc/installation/with-docker.en-us.md
+++ b/docs/content/doc/installation/with-docker.en-us.md
@@ -287,9 +287,18 @@ docker-compose up -d
 
 ## Managing Deployments With Environment Variables
 
-In addition to the environment variables above, any settings in `app.ini` can be set or overridden with an environment variable of the form: `GITEA__SECTION_NAME__KEY_NAME`. These settings are applied each time the docker container starts. Full information [here](https://github.com/go-gitea/gitea/tree/master/contrib/environment-to-ini).
-
-These environment variables can be passed to the docker container in `docker-compose.yml`. The following example will enable an smtp mail server if the required env variables `GITEA__mailer__FROM`, `GITEA__mailer__HOST`, `GITEA__mailer__PASSWD` are set on the host or in a `.env` file in the same directory as `docker-compose.yml`:
+In addition to the environment variables above, any settings in `app.ini` can be set
+or overridden with an environment variable of the form: `GITEA__SECTION_NAME__KEY_NAME`.
+These settings are applied each time the docker container starts.
+Full information [here](https://github.com/go-gitea/gitea/tree/master/contrib/environment-to-ini).
+
+These environment variables can be passed to the docker container in `docker-compose.yml`.
+The following example will enable an smtp mail server if the required env variables
+`GITEA__mailer__FROM`, `GITEA__mailer__HOST`, `GITEA__mailer__PASSWD` are set on the host
+or in a `.env` file in the same directory as `docker-compose.yml`.
+
+The settings can be also set or overridden with the content of a file by defining an environment variable of the form:
+`GITEA__section_name__KEY_NAME__FILE` that points to a file.
 
 ```bash
 ...