[PATCH 3/3] Added doc to setup apt cache.

["alexander.heinisch via isar-users" <isar-users@googlegroups.com>]

From: Alexander Heinisch <alexander.heinisch@siemens.com>

Signed-off-by: Alexander Heinisch <alexander.heinisch@siemens.com>
---
 doc/apt-cache.md | 55 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)
 create mode 100644 doc/apt-cache.md

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+# Using an APT Cache
+
+When working on the road limited download bandwitdth oftentimes is an issue, and increases the build times drastically.
+On the other hand large corporate networks could get rate limited by debian mirrors, as many peoply / pipelines / aso.
+fetch huge amounts of packets.
+
+An apt cache can mitigate these issues.
+
+## Setup
+
+1. Install apt cache
+
+```
+apt install apt-cache-ng
+```
+
+## Using the cache using premirrors
+
+1. Set `DISTRO_APT_PREMIRRORS` in your projects `bitbake` or `kas` config:
+
+```
+DISTRO_APT_PREMIRRORS="http://deb\.debian\.org http://localhost:3142/deb.debian.org"\n""
+```
+
+1. Invoke `kas menu` and select
+    1. `Mirror selection ->
+    1. Use specific (Debian | Ubuntu) mirror ->
+    1. URL of (Debian | Ubuntu) mirror (e.g. `http://localhost:3142/deb.debian.org`).
+
+
+## Using the cache when snapshot builds are configured
+
+When you chose to use isar's snapshot mechanism:
+```
+ISAR_USE_APT_SNAPSHOT = "1"
+```
+you can enable the cache, depending on your build environment, as follows:
+
+1. Set `DISTRO_APT_SNAPSHOT_PREMIRROR_BASE` in your projects `bitbake` or `kas` config:
+
+```
+DISTRO_APT_SNAPSHOT_PREMIRROR_BASE=localhost:3142/snapshot.debian.org
+```
+
+1. Set `DISTRO_APT_SNAPSHOT_PREMIRROR_BASE`  in your environment and include `kas/opt/mirror-snapshot.yaml` to your kas file or append it on kas cli invokation.
+
+1. Invoke `kas menu` and select
+    1. `Mirror selection ->
+    1. Use the distros snapshot mirror ->
+    1. Use a cache for snapshot mirror and specify the snapshot mirror you prefer (e.g. `localhost:3142/snapshot.debian.org`).
+
+
+> Note: When invoking the build in a containerized environment like `kas/kas-container menu`, you have to specify the ip address of the machine running `apt-cacher-ng`.
+
+> Note: To limit access to the `apt-cacher-ng`, a dummy network interface can help!
-- 
2.43.0

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