feat: allow setting DOCKER_BIN path for docker execution

This change allows operators to specify a DOCKER_BIN environment variable. This will specify a binary to run when executing docker, which is useful in cases where the 'docker' command being run must be modified in a way that would otherwise be invasive to Dokku, but minimalistic if done within a wrapper.

plugins/ps/subcommands/default

@@ -21,7 +21,7 @@ ps_main_cmd() {
     has_tty && local DOKKU_RUN_OPTS="-i -t"
     dokku_log_info1_quiet "running processes in container: $CID"
     # shellcheck disable=SC2086
-    docker exec $DOKKU_RUN_OPTS $CID /bin/sh -c "ps auxwww"
+    "$DOCKER_BIN" exec $DOKKU_RUN_OPTS $CID /bin/sh -c "ps auxwww"
   done
 }