feat(devcontainer): install prebuilt nektos/act executable

- Remove the now-unneeded gh CLI devcontainer feature.

.devcontainer/Dockerfile

@@ -28,3 +28,7 @@ RUN --mount=type=cache,target=/root/.cache poetry install
 FROM d3lta-prod AS d3lta-dev
 
 RUN --mount=type=cache,target=/root/.cache poetry install --with dev
+
+# install nektos/act as specified in https://nektosact.com/installation/index.html#bash-script
+# the -b flag specifies the target directory (cf. https://github.com/nektos/act/blob/61396d8085a9d812cebf94fa954f5938d48bf2b9/install.sh#L13)
+RUN curl --proto '=https' --tlsv1.2 -sSf https://raw.githubusercontent.com/nektos/act/master/install.sh | sudo bash -s -- -b /usr/bin

.devcontainer/devcontainer.json

@@ -11,6 +11,7 @@
 			]
 		}
 	},
-	// Features to add to the dev container. More info: https://containers.dev/features.
-	// "features": {}
+	"features": {
+		"ghcr.io/devcontainers/features/docker-in-docker:2": {}
+	}
 }