devops: add utility to count compressed docker image size (#2920)

This adds a new script to calculate docker image size with
all parent layers.

Note: take this metrics with a grain of salt, since in reality
docker compresses and reuses layers.

Some historic stats obtained with this script:
- **`208MB`** (-33MB) chore(docker): skip "recommended" dependencies (#2917) (1cebf87)
- **`241MB`** (-29MB) chore(docker): trim some of the gstreamer dependencies (#2897) (bce4b1a)
- **`272MB`** (-1MB) devops: do cache busting for APT (#2656) (bb34418)
- **`273MB`** (+49MB) fix(webkit): update Docker file to include gstreamer (#2636) (5c6c659)
- **`224MB`** (+0MB) chore: fix emojis for CR and FF in Dockerfile (#2522) (24316ad)
- **`224MB`** (-1MB) fix: Dockerfile for Firefox (#1937) (b516ac4)
- **`225MB`** (+49MB) devops(docker): Install ffmpeg dependency, adding codecs necessary for video playback in Firefox (#1627) (222d01c)
-  **`176MB`** (+32MB) chore(docs): optionally install XVFB in docker(ec3ee66)
-  **`144MB`** (+144MB)  feat: add a playwright-ready docker image (#1161)(1781ae7)

Author: aslushnikov

docs/docker/CURRENT_DOCKER_IMAGE_SIZE

@@ -0,0 +1,2 @@
+(generated with docker-image-size.sh)
+224M

docs/docker/docker-image-size.sh

@@ -0,0 +1,32 @@
+#!/bin/bash
+set -e
+set +x
+
+# This script computes **compressed image size with all its layers**.
+# This solution is based on https://stackoverflow.com/a/55156181/314883
+
+DOCKER_IMAGE_NAME="docker-image-to-count-compressed-size"
+FILE_NAME="docker-image-to-count-compressed-size"
+
+function cleanup() {
+  echo "-- Removing .tar if any"
+  rm -f "${FILE_NAME}.tar"
+  echo "-- Removing .tar.gz if any"
+  rm -f "${FILE_NAME}.tar.gz"
+  echo "-- Removing docker image if any"
+  docker rmi "${DOCKER_IMAGE_NAME}:bionic" >/dev/null
+}
+
+trap "cleanup; cd $(pwd -P)" EXIT
+cd "$(dirname "$0")"
+
+echo "-- Building image..."
+docker build -t "${DOCKER_IMAGE_NAME}:bionic" -f Dockerfile.bionic . >/dev/null
+echo "-- Saving .tar of the image..."
+docker save "${DOCKER_IMAGE_NAME}:bionic" > "${FILE_NAME}.tar"
+echo "-- Compressing image..."
+gzip "${FILE_NAME}.tar" >/dev/null
+
+echo "(generated with docker-image-size.sh)" > CURRENT_DOCKER_IMAGE_SIZE
+du -sh ${FILE_NAME}.tar.gz | cut -f1 | xargs >> CURRENT_DOCKER_IMAGE_SIZE
+