Merge pull request #2675 from dvdksn/run-mount-secret-env

docs: update run mount secrets examples using env

Author: crazy-max

docs/bake-reference.md

@@ -871,8 +871,8 @@ This lets you [mount the secret][run_mount_secret] in your Dockerfile.
 ```dockerfile
 RUN --mount=type=secret,id=aws,target=/root/.aws/credentials \
     aws cloudfront create-invalidation ...
-RUN --mount=type=secret,id=KUBECONFIG \
-    KUBECONFIG=$(cat /run/secrets/KUBECONFIG) helm upgrade --install
+RUN --mount=type=secret,id=KUBECONFIG,env=KUBECONFIG \
+    helm upgrade --install
 ```
 
 ### `target.shm-size`

docs/reference/buildx_build.md

@@ -947,8 +947,8 @@ Attribute keys:
 # syntax=docker/dockerfile:1
 FROM node:alpine
 RUN --mount=type=bind,target=. \
-  --mount=type=secret,id=SECRET_TOKEN \
-  SECRET_TOKEN=$(cat /run/secrets/SECRET_TOKEN) yarn run test
+  --mount=type=secret,id=SECRET_TOKEN,env=SECRET_TOKEN \
+  yarn run test
 ```
 
 ```console