docs: add a short container healthcheck page

content/manuals/engine/containers/healthchecks.md

@@ -0,0 +1,62 @@
+---
+title: Check a container's health
+description: Use HEALTHCHECK or docker run --health-cmd to mark a container healthy or unhealthy
+keywords: containers, healthcheck, HEALTHCHECK, docker run
+weight: 15
+---
+
+A health check is a command Docker runs inside the container to decide whether
+the process is actually serving. The container still has a normal status
+(`created`, `running`, `exited`). When a health check is set, it also has a
+health status:
+
+- `starting` — checks have not succeeded yet
+- `healthy` — the last check succeeded
+- `unhealthy` — too many checks in a row failed
+
+Compose `depends_on` with `condition: service_healthy` and Swarm wait for
+`healthy` before treating a replica as ready.
+
+## In a Dockerfile
+
+```dockerfile
+HEALTHCHECK --interval=5s --timeout=3s --start-period=20s --retries=3 \
+  CMD curl -fsS http://127.0.0.1:8080/health || exit 1
+```
+
+`--start-period` is grace time after the container starts. Failures in that
+window do not count toward `--retries`. A success during the start period
+ends it. `--retries` is consecutive failures (the first check counts) and
+must be 1 or higher; `0` is ignored and the daemon uses 3.
+
+There can be only one `HEALTHCHECK`. A later one replaces the earlier one.
+`HEALTHCHECK NONE` disables a check inherited from the base image.
+
+See the [Dockerfile reference](/reference/dockerfile.md#healthcheck).
+
+## On `docker run`
+
+Flags override the image's `HEALTHCHECK`:
+
+```console
+$ docker run -d --name web \
+    --health-cmd='curl -fsS http://127.0.0.1:8080/health || exit 1' \
+    --health-interval=5s \
+    --health-retries=3 \
+    --health-start-period=20s \
+    nginx:alpine
+```
+
+`--health-cmd` always runs through `CMD-SHELL`. Use
+`--no-healthcheck` to turn the image check off.
+
+See [`docker run`](/reference/cli/docker/container/run/).
+
+## Read the status
+
+```console
+$ docker inspect --format='{{.State.Health.Status}}' web
+healthy
+```
+
+`docker ps` shows `(healthy)` next to the status when a check is configured.