Ignore .git/ dir when copying files for Docker build

This can manifest as errors during tests attempty to do a local Git
clone:

```
34.36 2024-08-06T10:46:26Z      DEBUG   controller-runtime.cluster.events       failed setting up repository clone: Run(/usr/bin/git clone --no-checkout /src /tmp/run_app-a_app-a_repo_380715592): exit status 128: { stdout: "", stderr: "Cloning into '/tmp/
run_app-a_app-a_repo_380715592'...\nfatal: hardlink different from source at '/tmp/run_app-a_app-a_repo_380715592/.git/objects/pack/pack-3eb4972719122450751fc1840c8260f9393efd5b.idx'\n" }     {"type": "Warning", "object": {"kind":"Waybill","namespace":"ap
p-a","name":"app-a","uid":"4063ba1b-fa52-4605-b3cf-0c0ace497b1b","apiVersion":"kube-applier.io/v1alpha1","resourceVersion":"202"}, "reason": "WaybillRunRequestFailed"}
``

.dockerignore

@@ -0,0 +1 @@
+.git/