chore: test heredoc syntax support

Author: devthejo

test/__snapshots__/HEREDOC/heredoc.expected.dockerfile

@@ -0,0 +1,14 @@
+FROM debian:latest
+ARG DEBUG
+SHELL ["/bin/bash", "-lc"]
+# Heredoc with preserved indentation and continuations
+RUN <<'EOF'
+[ -z "$DEBUG" ] || set -ex && set -e
+if command -v something; then
+    something \
+        --arg1 test1 \
+        --arg2 test2
+else
+    echo "something not found"
+fi
+EOF

test/features.js

@@ -40,3 +40,9 @@ describe("COPY", () => {
     await testFixture("copyFromStage")
   })
 })
+
+describe("HEREDOC", () => {
+  it("heredoc", async () => {
+    await testFixture("heredoc")
+  })
+})

test/fixtures/heredoc.dockerfile

@@ -0,0 +1,16 @@
+# syntax = devthefuture/dockerfile-x:1.4.3
+FROM debian:latest
+ARG DEBUG
+SHELL ["/bin/bash", "-lc"]
+
+# Heredoc with preserved indentation and continuations
+RUN <<'EOF'
+[ -z "$DEBUG" ] || set -ex && set -e
+if command -v something; then
+    something \
+        --arg1 test1 \
+        --arg2 test2
+else
+    echo "something not found"
+fi
+EOF