Add override for container tool in dev_container script

This change allows to override the command line tool that is used to
start the dev container in the `tools/dev_container` script. The
Makefile for creating the dev container uses the `DOCKER` env var to
implement this, so this is used here as well.

Rationale: Googlers are advised to not install Docker
(go/dont-install-docker) and to use Podman instead.

If the `DOCKER` variable is unset, the script will try to use
Docker first and podman.

Change-Id: I33bc8e4af632fa982406e838e762572407ccdc22
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3259939
Reviewed-by: Dennis Kempin <denniskempin@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Christian Blichmann <cblichmann@google.com>

tools/dev_container

@@ -6,7 +6,7 @@
 set -e
 cd "$(dirname $0)/.."
 
-docker_args=(
+cli_args=(
     --rm
     --volume $(pwd):/workspace:rw
     --device /dev/net/tun
@@ -17,11 +17,15 @@ docker_args=(
 
 # Enable interactive mode when running in an interactive terminal.
 if [ -t 1 ]; then
-    docker_args+=(-it)
+    cli_args+=(-it)
 fi
 
-docker run \
-    ${docker_args[@]} \
+# Allow to override the container CLI tool, similar to the Makefile. Try
+# "docker" first and fall back to "podman".
+DOCKER=${DOCKER:-$(which docker || which podman)}
+
+"${DOCKER}" run \
+    ${cli_args[@]} \
     gcr.io/crosvm-packages/crosvm_dev:$(cat tools/impl/dev_container/version) \
     "$@"