Add fedora and alpine release builds

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

.gitlab-ci.yml

@@ -239,7 +239,7 @@ build-docker-tag:
 
 # Packaging app for amd64
 package-app:
-  image: hexpm/elixir:1.13.4-erlang-24.3.2-${OS}
+  image: 1.13.4-erlang-24.3.3-${OS}
   stage: package
   before_script:
     - apt-get update && apt-get install -yq build-essential git curl cmake
@@ -268,7 +268,7 @@ package-app:
       - ${APP_ASSET}
   parallel:
     matrix:
-      - OS: ["debian-bullseye-20210902", "debian-buster-20210902", "ubuntu-focal-20211006", "ubuntu-bionic-20210930"]
+      - OS: ["debian-bullseye", "debian-buster", "ubuntu-focal", "ubuntu-bionic", "fedora-35", "fedora-36", "alpine-3.14.5", "alpine-3.15.3"]
 
 package-app-dev:
   stage: package

UPGRADE.md

@@ -1,5 +1,26 @@
 # Upgrading from 2.0 to 2.1
 
+## Distro-specific packages
+
+Elixir releases are precompiled binaries and are therefore dependent on the host they've been compiled to be compatible with your system (architecture, libc version, …) ([read more](https://hexdocs.pm/mix/Mix.Tasks.Release.html#module-requirements)). For instance, the release build on Debian Bullseye (with libc 2.31) cannot be run on Debian Buster (which only provides libc 2.28). Another example is Red Hat-relative distributions (Fedora, CentOS, …) which have an OpenSSL library version that's not compatible with the version compiled (because of [potential patent issues with Elliptic curves](https://github.com/kerl/kerl#compiling-crypto-on-red-hat-systems)).
+
+In order to make sure the releases package we redistribute , we now build distro-version-specific packages.
+The list of packages built for version 2.1.0 with the amd64 (x86_64) architecture is the following:
+  - Debian Buster (10)
+  - Debian Bullseye (11)
+  - Ubuntu Bionic (18.04)
+  - Ubuntu Focal (20.04)
+  - Fedora 35
+  - Fedora 36
+  - Alpine 3.14.5
+  - Alpine 3.15.3
+
+As building for non-`amd64` architectures is quite difficult (because of the time and extra resources cross-compiling takes), these architectures are only built against Debian Bullseye (11) at the moment.
+
+We want to expand this list in the future to make sure it covers most uses, so feel free to give us feedback from what you need.
+
+Note: Docker images are also built for `amd64`/`arm`/`arm64` architectures, and of course you can always install Mobilizon from source on pretty much any unix-compatible system.
+
 ## Mailer library change
 
 ### Docker