devcontainer: introduce VSCode dev container

Introduce Dockerfile and devcontainer.json specifying how to build and
run a containerized development environment in VSCode.

The configuration is inspired by the osbuild repo where a similar
configuration directory already exists.

.devcontainer/devcontainer.json

@@ -0,0 +1,11 @@
+{
+  "name": "ComposerBuilder",
+  "build": {
+    "dockerfile": "Dockerfile",
+    "context": ".."
+  },
+  "extensions": [
+    "laurenttreguier.rpm-spec",
+    "golang.Go"
+  ]
+}