.dockerignore: exclude files from docker copying

Exclude the .git directory and some other directories that aren't
needed to build the worker.

This reduces the total amount of data sent to the docker daemon
to from 544M to 355M.

Change-Id: Ic6f41f6c86f1776e056e881ef67c750137e17a7e
Reviewed-on: https://go-review.googlesource.com/c/pkgsite-metrics/+/471161
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Maceo Thompson <maceothompson@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
diff --git a/.dockerignore b/.dockerignore
new file mode 100644
index 0000000..a5a6540
--- /dev/null
+++ b/.dockerignore
@@ -0,0 +1,5 @@
+**/.git
+devtools
+deploy
+terraform
+