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.

tests/unit/40_report.bats

@@ -5,9 +5,11 @@ load test_helper
 setup() {
   global_setup
   deploy_app
+  rm -f /tmp/fake-docker-bin
 }
 
 teardown() {
+  rm -f /tmp/fake-docker-bin
   destroy_app
   global_teardown
 }
@@ -46,3 +48,33 @@ teardown() {
 
   dokku --force apps:destroy "${TEST_APP}-2"
 }
+
+@test "(report) custom docker bin" {
+  export DOCKER_BIN="docker"
+  run /bin/bash -c "dokku report"
+  echo "output: $output"
+  echo "status: $status"
+  assert_output_contains "herokuish version:"
+  assert_success
+
+  export DOCKER_BIN="/usr/bin/docker"
+  run /bin/bash -c "dokku report"
+  echo "output: $output"
+  echo "status: $status"
+  assert_output_contains "herokuish version:"
+  assert_success
+
+  touch /tmp/fake-docker-bin
+  echo '#!/usr/bin/env bash' >/tmp/fake-docker-bin
+  echo '/usr/bin/docker "$@"' >>/tmp/fake-docker-bin
+  chmod +x /tmp/fake-docker-bin
+
+  export DOCKER_BIN="/tmp/fake-docker-bin"
+  run /bin/bash -c "dokku report"
+  echo "output: $output"
+  echo "status: $status"
+  assert_output_contains "herokuish version:"
+  assert_success
+
+  unset DOCKER_BIN
+}