Add asdf install guide for debian/ubuntu

Closes #557

docs/docs/installation/debian_based_en.md

@@ -23,23 +23,7 @@ sudo apt full-upgrade
 sudo apt install git build-essential postgresql postgresql-contrib cmake libmagic-dev
 ```
 
-### Install Elixir and Erlang
-
-* Install Elixir and Erlang (you might need to use backports or [asdf](https://github.com/asdf-vm/asdf) on old systems):
-
-```shell
-sudo apt update
-sudo apt install elixir erlang-dev erlang-nox
-```
-
-
-### Optional packages: [`docs/installation/optional/media_graphics_packages.md`](../installation/optional/media_graphics_packages.md)
-
-```shell
-sudo apt install imagemagick ffmpeg libimage-exiftool-perl
-```
-
-### Install AkkomaBE
+### Create the akkoma user
 
 * Add a new system user for the Akkoma service:
 
@@ -49,7 +33,61 @@ sudo useradd -r -s /bin/false -m -d /var/lib/akkoma -U akkoma
 
 **Note**: To execute a single command as the Akkoma system user, use `sudo -Hu akkoma command`. You can also switch to a shell by using `sudo -Hu akkoma $SHELL`. If you don’t have and want `sudo` on your system, you can use `su` as root user (UID 0) for a single command by using `su -l akkoma -s $SHELL -c 'command'` and `su -l akkoma -s $SHELL` for starting a shell.
 
-* Git clone the AkkomaBE repository from stable-branch and make the Akkoma user the owner of the directory:
+### Install Elixir and Erlang
+
+If your distribution packages Elixir 1.14+, you can install it directly from the distro repositories and skip to the next section of the guide:
+
+```shell
+sudo apt install elixir erlang-dev erlang-nox
+```
+
+Otherwise use [asdf](https://github.com/asdf-vm/asdf) to install the latest versions of Elixir and Erlang.
+
+First, install some dependencies needed to build Elixir and Erlang:
+```shell
+sudo apt install curl unzip build-essential autoconf m4 libncurses5-dev libwxgtk3.0-gtk3-dev libwxgtk-webview3.0-gtk3-dev libgl1-mesa-dev libglu1-mesa-dev libpng-dev libssh-dev unixodbc-dev xsltproc fop libxml2-utils libncurses-dev openjdk-11-jdk
+```
+
+Then login to the `akkoma` user and install asdf:
+```shell
+git clone https://github.com/asdf-vm/asdf.git ~/.asdf --branch v0.11.3
+```
+
+Add the following lines to `~/.bashrc`:
+```shell
+. "$HOME/.asdf/asdf.sh"
+# asdf completions
+. "$HOME/.asdf/completions/asdf.bash"
+```
+
+Next install Erlang:
+```shell
+asdf plugin add erlang https://github.com/asdf-vm/asdf-erlang.git
+asdf install erlang 25.3.2.1
+asdf global erlang 25.3.2.1
+```
+
+Now install Elixir:
+```shell
+asdf plugin-add elixir https://github.com/asdf-vm/asdf-elixir.git
+asdf install elixir 1.14.5-otp-25
+asdf global elxir 1.14.5-otp-25
+```
+
+Confirm that Elixir is installed correctly by checking the version:
+```shell
+elixir --version
+```
+
+### Optional packages: [`docs/installation/optional/media_graphics_packages.md`](../installation/optional/media_graphics_packages.md)
+
+```shell
+sudo apt install imagemagick ffmpeg libimage-exiftool-perl
+```
+
+### Install AkkomaBE
+
+* Log into the `akkoma` user and clone the AkkomaBE repository from the stable branch and make the Akkoma user the owner of the directory:
 
 ```shell
 sudo mkdir -p /opt/akkoma

installation/akkoma.service

@@ -8,6 +8,8 @@ Restart=on-failure
 
 ; Uncomment this if you're on Arch Linux
 ; Environment="PATH=/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl"
+; Uncomment if using asdf to manage Elixir and Erlang
+; Environment="PATH=/var/lib/akkoma/.asdf/shims:/var/lib/akkoma/.asdf/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
 
 ; Name of the user that runs the Akkoma service.
 User=akkoma
@@ -24,6 +26,8 @@ Environment="HOME=/var/lib/akkoma"
 WorkingDirectory=/opt/akkoma
 ; Path to the Mix binary.
 ExecStart=/usr/bin/mix phx.server
+; If using asdf comment the above line and uncomment the one below instead
+; ExecStart=/var/lib/akkoma/.asdf/shims/mix phx.server
 
 ; Some security directives.
 ; Use private /tmp and /var/tmp folders inside a new file system namespace, which are discarded after the process stops.