Added tests for workdir interpolation

Author: toote

tests/command.bats

@@ -214,6 +214,25 @@ setup() {
   unstub docker
 }
 
+@test "Runs BUILDKITE_COMMAND with workdir with variables and option turned on" {
+  # shellcheck disable=2016  # we want the variable not interpreted now
+  export BUILDKITE_PLUGIN_DOCKER_WORKDIR='$ONE_VAR'
+  export BUILDKITE_PLUGIN_DOCKER_EXPAND_VOLUME_VARS=true
+  export BUILDKITE_COMMAND="pwd"
+
+  export ONE_VAR=/my/path
+
+  stub docker \
+    "run -t -i --rm --init --volume $PWD:/my/path --workdir /my/path --label com.buildkite.job-id=1-2-3-4 image:tag /bin/sh -e -c 'pwd' : echo ran command in docker"
+
+  run "$PWD"/hooks/command
+
+  assert_success
+  assert_output --partial "ran command in docker"
+
+  unstub docker
+}
+
 @test "Runs BUILDKITE_COMMAND with devices" {
   export BUILDKITE_PLUGIN_DOCKER_WORKDIR=/app
   export BUILDKITE_PLUGIN_DOCKER_DEVICES_0=/dev/bus/usb/001/001