Document about the environment where we run all tests

Instead of moving things to pipeline.yml, this change explains more
about the EC2 instance we are using.

Author: kzys

.buildkite/pipeline.yml

@@ -12,6 +12,11 @@
 # permissions and limitations under the License.
 
 steps:
+  - label: ':ec2: environment'
+    command:
+      - uname -a
+      - go version
+
   - label: ':go: go mod download'
     command: 'go mod download'
     agents:

HACKING.md

@@ -43,3 +43,17 @@ following:
 1. Update `client/swagger.yaml`
 3. Run `go generate`
 4. Figure out what broke and fix it.
+
+Continuous Integration
+---
+
+To access `/dev/kvm`, we are using [BuildKite](https://buildkite.com/) and
+Amazon EC2 Bare Metal Instances.
+
+The instance is pre-configured to have
+
+  - firecracker and jailer under /usr/local/bin.
+    Both of them are currently v0.17.0 and built with
+    `cargo build --features vsock --release` to enable vsock support
+  - vmlinux.bin from
+    https://s3.amazonaws.com/spec.ccfc.min/img/hello/kernel/hello-vmlinux.bin