devcontainer: add doc, user to match host machine, and scripts to build/start

ops/devcontainer/start.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+sudo docker run --rm -it \
+    -v "$(pwd)":/code \
+    -v "$(go env GOCACHE):/gocache-vol" \
+    -e GOCACHE=/gocache-vol \
+    -v "$(go env GOMODCACHE):/gocache-vol/mod-cache" \
+    -e GOMODCACHE=/gocache-vol/mod-cache \
+    -e GOLANGCI_LINT_CACHE=/gocache-vol/lint-cache \
+    --workdir /code \
+    --net host \
+    gas