devcontainer: Add a dockerignore (#43127)

Rename the dockerfile and add a dockerignore. This reduces the size of
the build-context greatly.
Renaming is a preparation for later adding more possible dockerfiles
(e.g. fedora based etc).
We choose the filename based `.dockerignore` files over a
`.dockerignore` in the root folder mainly to reduce clutter in the root
folder.
This addresses the problem of large build context raised on zulip by
@glyn.

Testing: Manually tested by building the image locally.

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

.devcontainer/Ubuntu.Dockerfile.dockerignore

@@ -0,0 +1,3 @@
+*
+!/.devcontainer
+!/python/servo/platform/linux_packages

.devcontainer/devcontainer.json

@@ -3,7 +3,7 @@
 {
 	"name": "Ubuntu",
 	"build": {
-		"dockerfile": "Dockerfile",
+		"dockerfile": "Ubuntu.Dockerfile",
 		"context": ".."
 	},