docs: mention .pants.bootstrap file (#20538)

The `.pants.bootstrap` file is not mentioned anywhere, but it can be
helpful when declaring environment variables that are going to be
accessible both in `pants.toml` to interpolate and any subprocess Pants
may start (such as when doing `pants run script.py` and there's
`os.getenv("ENVVAR")`).

Author: AlexTereshenkov

docs/docs/using-pants/key-concepts/options.mdx

@@ -102,9 +102,13 @@ domain = "my.domain"
 
 [python-repos]
 repo_host = "repo.%(domain)s"
-indexes.add = ["http://%(env.PY_REPO)s@%(repo_host)s/index
+indexes.add = ["https://%(env.PY_REPO)s@%(repo_host)s/index"]
 ```
 
+Learn more about exporting environment variables in the
+[`.pants.bootstrap`](../using-pants/key-concepts/options.mdx#pantsbootstrap-file)) Bash script that is sourced
+before Pants runs.
+
 ## Option types
 
 Every option has a type, and any values you set must be of that type.
@@ -369,3 +373,25 @@ interpreter_constraints = ["==3.9.*"]
 ```
 
 If you want to ban this feature, set `[GLOBAL].pantsrc = false` in `pants.toml`.
+
+## `.pants.bootstrap` file
+
+If you need to set default values for environment variables without requiring all users to define them in the local
+environment, you can export them in the `.pants.bootstrap` Bash script. This file needs to be placed in the root of your
+workspace, and it is going to be sourced before invoking any Pants goal.
+
+You can also add to this file any Bash code you want to execute before Pants runs, and any environment variables
+declared in this file are going to be available to any process that Pants launcher binary may start.
+
+For example:
+
+```bash title=".pants.bootstrap"
+# these variables are defined in our CI agents,
+# but this is set to support local development
+export DOCKER_DEFAULT_REPO="https://hub.docker.com/"
+export GIT_COMMIT="$(git rev-parse HEAD)"
+```
+
+If you want to learn more about how
+[Pants launcher binary](../../getting-started/installing-pants#the-pants-binarys-implementation) works, see
+the [scie-pants](https://github.com/pantsbuild/scie-pants) project.

docs/docs/using-pants/troubleshooting-common-issues.mdx

@@ -206,7 +206,7 @@ This can be fixed by setting `ulimit -n 10000`. (10,000 should work in all cases
 :::note Tip: permanently configuring `ulimit -n`
 We recommend permanently setting this by either:
 
-1. Adding `ulimit -n 10000` to your `pants.bootstrap` script.
+1. Adding `ulimit -n 10000` to your [`.pants.bootstrap`](../using-pants/key-concepts/options.mdx#pantsbootstrap-file) script.
 2. Adding `ulimit -n 10000` to your global `.bashrc` or equivalent.
 
 The first two approaches have the benefit that they will be checked into version control, so every developer at your organization can use the same setting.