Fix #1684 (#2234)

Previous situation: we do most of our "check minimum requirements" work in the aptly named check-minimum-requirements.sh. This would be a natural home for the "check if docker even exists on this system" verification, but we actually run call into docker info to set some environment variables before this script gets run, in detect-platforms.sh. So I've adde the "does docker exist" check to that file, directly before we call docker info.

Author: azaslavsky

install/detect-platform.sh

@@ -12,8 +12,12 @@ echo "${_group}Detecting Docker platform"
 # linux/amd64 by default due to virtualization.
 # See https://github.com/docker/cli/issues/3286 for the Docker bug.
 
-export DOCKER_ARCH=$(docker info --format '{{.Architecture}}')
+if ! command -v docker &>/dev/null; then
+  echo "FAIL: Could not find a \`docker\` binary on this system. Are you sure it's installed?"
+  exit 1
+fi
 
+export DOCKER_ARCH=$(docker info --format '{{.Architecture}}')
 if [[ "$DOCKER_ARCH" = "x86_64" ]]; then
   export DOCKER_PLATFORM="linux/amd64"
   export CLICKHOUSE_IMAGE="yandex/clickhouse-server:20.3.9.70"