Set build context directory in devcontainer.json

Otherwise the default build context is the top level directory when building the dev container in JetBrain IDEs. VsCode seems to use the Dockerfile location as build context instead, meaning the container build works by default here.

Author: dariusarnold

.devcontainer.json

@@ -2,7 +2,8 @@
 // https://github.com/microsoft/vscode-dev-containers/tree/v0.154.2/containers/cpp
 {
 	"build": {
-		"dockerfile": "docker/Dockerfile"
+		"dockerfile": "docker/Dockerfile",
+		"context": "docker",
 	},
 	"customizations": {
 		"vscode": {