chore(linux): Update docs for new changes with urunit

Update the tutorail for running existing containers over Linux in urunc
in order to include the new changes on passing information to the guest
from the host.

Signed-off-by: Charalampos Mainas <cmainas@nubificus.co.uk>

Author: cmainas

docs/tutorials/existing-container-linux.md

@@ -14,7 +14,7 @@ machine (VM).
 Overall, we need to do the followings:
 
 1. Build or reuse a Linux kernel.
-2. (Optional) Build or fetch an init process.
+2. (Optional but recommended) Build or fetch an init process.
 3. Prepare the final image by appending the Linux kernel (and init) and set up
    `urunc` annotations.
 
@@ -54,25 +54,61 @@ However, this is not always reliable:
 - If the application exits, the system halts.
 - CLI argument handling may be incorrect: Linux does not natively support
   multi-word arguments via kernel boot parameters. Each space-separated word is
-  treated as a separate argument.
+treated as a separate argument.
 - A few necessary operations (such as mount /proc, set default route) might be
   required before executing the application.
+- The Linux kernel can only handle a limited number of characters in its boot
+  parameters. If too many or very long environment variables are passed, the
+  boot process will fail once that limit is exceeded. 
 
 Especially, for CLI argument handing, `urunc` follows a simple convention. All
 multi-word CLI arguments are wrapped in single quotes and the init process (or
 application) is expected to reconstruct them properly.
 
-For all the above reasons, we recommend using a dedicated init process. We
-provide [urunit](https://github.com/nubificus/urunit#); a lightweight init
+For all the above reasons, we strongly recommend using a dedicated init process.
+We provide [urunit](https://github.com/nubificus/urunit#); a lightweight init
 designed specifically for `urunc`. It performs the following actions:
 
 1. Sets default route through eth0. This is necessary when we deploy
    the container in a Kubernetes cluster, where there is a high chance that
    the gateway of the container might be in a different subnet than the IP.
    As a result, Linux kernel will fail to set the gateway.
 2. Groups multi-word arguments correctly.
+3. Mounts `/proc` and `/sys`.
+3. Reads and sets the environment variables for the application execution
+3. Reads and sets the uid,gid and working directory for the application execution
 3. Acts as a reaper, cleaning up zombie processes.
 
+To pass the necessary infomration to urunitlink, `urunc` uses a configuration
+file that passes to the VM with the following format:
+
+```
+UES
+/* list of environment variables */
+UEE
+UCS
+UID:<uid>
+GID:<gid>
+WD:<working_dir>
+UCE
+```
+
+In order to minimize the dependencies for the Linux kernel running as guest,
+`urunc` attaches this configuration file as an initrd of the VM and sets the
+`retain_initrd` kernel boot parameter. In that way, urunitlink will be able to
+read the configuration file from `/sys/firmware/initrd`. In case the guest is
+configured to boot with a user-specified initrd, then `urunc` will archive
+in the cpio format the configuration file and concatenate it with the
+user-provided initrd. The Linux kernel is smart enough to handle concatenated
+initrd and hence the configuration file will appear in the guest's rootfs.
+
+As a result, in case urunitlink is used we strongly recommend to configure
+the Linux kernel setting the following configuration options:
+
+- Support for initrd `CONFIG_BLK_DEV_INITRD`
+- Support for sysfs `CONFIG_SYSFS`
+- Support for procfs `CONFIG_PROC_FS`
+
 You can obtain [urunit](https://github.com/nubificus/urunit) in two ways:
 
 - Fetch a static binary from [urunit's release