add some more tests for custom image config

Author: jeremybumsted

tests/run.bats

@@ -225,3 +225,22 @@ load "${BATS_PLUGIN_PATH}/load.bash"
 
   unstub docker
 }
+
+
+@test "Shellcheck uses default image when empty string set" {
+  export BUILDKITE_PLUGIN_SHELLCHECK_FILES_0="tests/testdata/test.sh"
+  export BUILDKITE_PLUGIN_SHELLCHECK_IMAGE=""
+  export BUILDKITE_PLUGIN_SHELLCHECK_VERSION="stable"
+
+  stub docker \
+    "run --rm -v $PWD:/mnt --workdir /mnt koalaman/shellcheck:stable --color=always tests/testdata/test.sh : echo running shellcheck on test.sh"
+
+  run "$PWD/hooks/command"
+
+  assert_success
+  assert_output --partial "Running shellcheck on 1 files"
+  assert_output --partial "running shellcheck on test.sh"
+  assert_output --partial "Files are ok"
+
+  unstub docker
+}