docker: add Dockerfile and .dockerignore

Change-Id: I81b491e66a81c5b66c1561a9484434267e9b544d
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5140
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/.dockerignore b/.dockerignore
new file mode 100644
index 0000000..fbd11a8
--- /dev/null
+++ b/.dockerignore
@@ -0,0 +1,15 @@
+*
+*/
+
+# not ignore go and cue files
+!cmd/
+!cue/
+!cuego/
+!encoding/
+!internal/
+!pkg/
+!tools/
+
+# not ignore go module files
+!go.mod
+!go.sum