Add npm to apt packages.

Missed nodejs vs npm in https://skia-review.googlesource.com/c/buildbot/+/430060

Change-Id: I64a9677c43e9d77dfaf88d6a9bdb9e0d66d32b11
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/430436
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
diff --git a/docker/Dockerfile b/docker/Dockerfile
index 0123842..ca0b22c 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -11,6 +11,7 @@
   curl \
   clang \
   nodejs \
+  npm \
   wget
 
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