Add ffmpeg tool

Such tool is mandatory for videorecordering.
Also add mandatory ENV variable FFMPEG_INSTALLED.

Related-prod: PRODX-9091
Change-Id: Ia495463091f0f45e1acb46a9d3362530c97a5d74
diff --git a/Dockerfile b/Dockerfile
index 93b79b7..bbe88af 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -15,12 +15,14 @@
     zlib1g-dev \
     python3-pip \
     firefox \
+    ffmpeg \
     xvfb \
     wget \
     git \
     x11-xkb-utils
 
 ENV SELENIUM_HEADLESS=1
+ENV FFMPEG_INSTALLED=True
 
 RUN wget https://artifactory.mcp.mirantis.net/artifactory/binary-dev-local/tarballs/geckodriver-v0.24.0-linux64.tar.gz
 RUN tar xvzf geckodriver-v0.24.0-linux64.tar.gz && \