Adjusted MediaProxyControllerTest to gracefully fail on missing dependencies. Installation docs update.

Added ffmpeg/imagemagick checks to launch checks (if media preview proxy is enabled). Added documentation on installing optional media / graphics packages (imagemagick, ffmpeg, exiftool).

docs/installation/alpine_linux_en.md

@@ -13,8 +13,6 @@ It assumes that you have administrative rights, either as root or a user with [s
 * `erlang-parsetools`
 * `erlang-xmerl`
 * `git`
-* `ffmpeg`
-* `ImageMagick`
 * Development Tools
 * `cmake`
 
@@ -22,6 +20,9 @@ It assumes that you have administrative rights, either as root or a user with [s
 
 * `nginx` (preferred, example configs for other reverse proxies can be found in the repo)
 * `certbot` (or any other ACME client for Let’s Encrypt certificates)
+* `ImageMagick`
+* `ffmpeg`
+* `exiftool`
 
 ### Prepare the system
 
@@ -31,7 +32,6 @@ It assumes that you have administrative rights, either as root or a user with [s
 awk 'NR==2' /etc/apk/repositories | sed 's/main/community/' | tee -a /etc/apk/repositories
 ```
 
-
 * Then update the system, if not already done:
 
 ```shell
@@ -59,12 +59,6 @@ sudo apk add erlang erlang-runtime-tools erlang-xmerl elixir
 sudo apk add erlang-eldap
 ```
 
-### Install ffmpeg and ImageMagick
-
-```shell
-sudo apk add ffmpeg imagemagick
-```
-
 ### Install PostgreSQL
 
 * Install Postgresql server:
@@ -85,6 +79,12 @@ sudo /etc/init.d/postgresql start
 sudo rc-update add postgresql
 ```
 
+### Install media / graphics packages (optional, see [`docs/installation/optional/media_graphics_packages.md`](docs/installation/optional/media_graphics_packages.md))
+
+```shell
+sudo apk add ffmpeg imagemagick exiftool
+```
+
 ### Install PleromaBE
 
 * Add a new system user for the Pleroma service: