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.

dokku

@@ -2,6 +2,7 @@
 set -eo pipefail
 shopt -s nullglob
 
+export DOCKER_BIN=docker
 if [[ -r /etc/default/dokku ]]; then
   # shellcheck disable=SC1091
   source /etc/default/dokku
@@ -36,6 +37,10 @@ DOKKU_DISTRO=$(
   echo "$ID"
 )
 
+# Specify twice so that only host-level
+# configuration can ever override the DOCKER_BIN value
+export DOCKER_BIN=${DOCKER_BIN:="docker"}
+
 export DOKKU_IMAGE=${DOKKU_IMAGE:="gliderlabs/herokuish:latest"}
 export DOKKU_LIB_ROOT=${DOKKU_LIB_PATH:="/var/lib/dokku"}