local test instructions fail trying to copy the collector build

[tsloughter]

In order to get the docs in run-mock-test.md to work I found I either needed to first run:

git clone git@github.com:aws-observability/aws-otel-collector.git
cd aws-otel-collector
make build
cd ..

To build the collector or to patch the local/docker_compose.tpl file:

-        BUILDMODE: copy
+        BUILDMODE: build

Since I wasn't sure which solution you preferred I'm opening an issue instead of a PR but I can open a PR to either update the docs or update the template to build instead of copy the collector.