variable interpolation note

README.md

@@ -15,7 +15,7 @@ steps:
   - command: "go build -o dist/my-app ."
     artifact_paths: "./dist/my-app"
     plugins:
-      - docker#v5.8.0:
+      - docker#v5.9.0:
           image: "golang:1.11"
 ```
 
@@ -25,7 +25,7 @@ Windows images are also supported:
 steps:
   - command: "dotnet publish -c Release -o published"
     plugins:
-      - docker#v5.8.0:
+      - docker#v5.9.0:
           image: "microsoft/dotnet:latest"
           always-pull: true
 ```
@@ -37,7 +37,7 @@ If you want to control how your command is passed to the docker container, you c
 ```yml
 steps:
   - plugins:
-      - docker#v5.8.0:
+      - docker#v5.9.0:
           image: "mesosphere/aws-cli"
           always-pull: true
           command: ["s3", "sync", "s3://my-bucket/dist/", "/app/dist"]
@@ -52,7 +52,7 @@ Note: If you are utilizing Buildkite's [Elastic CI Stack S3 Secrets plugin](http
 steps:
   - command: "yarn install; yarn run test"
     plugins:
-      - docker#v5.8.0:
+      - docker#v5.9.0:
           image: "node:7"
           always-pull: true
           environment:
@@ -70,7 +70,7 @@ steps:
     env:
       MY_SPECIAL_BUT_PUBLIC_VALUE: kittens
     plugins:
-      - docker#v5.8.0:
+      - docker#v5.9.0:
           image: "node:7"
           always-pull: true
           propagate-environment: true
@@ -84,7 +84,7 @@ steps:
     env:
       MY_SPECIAL_BUT_PUBLIC_VALUE: kittens
     plugins:
-      - docker#v5.8.0:
+      - docker#v5.9.0:
           image: "node:7"
           always-pull: true
           propagate-aws-auth-tokens: true
@@ -96,7 +96,7 @@ You can pass in additional volumes to be mounted. This is useful for running Doc
 steps:
   - command: "docker build . -t image:tag; docker push image:tag"
     plugins:
-      - docker#v5.8.0:
+      - docker#v5.9.0:
           image: "docker:latest"
           always-pull: true
           volumes:
@@ -109,12 +109,41 @@ You can disable the default behaviour of mounting in the checkout to `workdir`:
 steps:
   - command: "npm start"
     plugins:
-      - docker#v5.8.0:
+      - docker#v5.9.0:
           image: "node:7"
           always-pull: true
           mount-checkout: false
 ```
 
+Variable interpolation can be tricky due to the 3 layers involved (Buildkite, agent VM, and docker). For example, if you want to use [ECR Buildkite plugin](https://github.com/buildkite-plugins/ecr-buildkite-plugin), you will need to use the following syntax. Note the `$$` prefix for variables that would otherwise resolve at pipeline upload time, not runtime:
+
+```yml
+steps:
+  - command: "yarn install; yarn run test"
+    plugins:
+      - ecr#v2.7.0:
+          login: true
+          account_ids:
+          - "d"
+          - "p"
+          region: us-west-2
+          no-include-email: true
+      - docker#v5.9.0:
+          image: "d.dkr.ecr.us-west-2.amazonaws.com/imagename"
+          command: ["./run-integration-tests.sh"]
+          expand-volume-vars: true
+          volumes:
+            - "/var/run/docker.sock:/var/run/docker.sock"
+            - "$$BUILDKITE_DOCKER_CONFIG_TEMP_DIRECTORY/config.json:/root/.docker/config.json"
+          propagate-environment: true
+          environment:
+            - "BUILDKITE_DOCKER_CONFIG_TEMP_DIRECTORY"
+
+```
+
+You can read more about runtime variable interpolation from the [docs](https://buildkite.com/docs/pipelines/environment-variables#runtime-variable-interpolation).
+
+
 ### `load`
 
 If the image that you want to run is not in a registry the `load` property can be used to load an image from a tar file.
@@ -127,12 +156,11 @@ steps:
     plugins:
       - artifacts#v1.9.0:
           download: "node-7-image.tar.gz"
-      - docker#v5.8.0:
+      - docker#v5.9.0:
           load: "node-7-image.tar.gz"
           image: "node:7"
 ```
 
-
 ### 🚨 Warning
 
 You need to be careful when/if [running the BuildKite agent itself in docker](https://buildkite.com/docs/agent/v3/docker) that, itself, runs pipelines that use this plugin. Make sure to read all the documentation on the matter, specially the caveats and warnings listed.