adr: add env context (actions#278)

docs/adrs/0278-env-context.md

@@ -0,0 +1,51 @@
+# ADR 0278: Env Context
+
+**Date**: 2019-09-30
+
+**Status**: Accepted
+
+## Context
+
+User wants to reference workflow variables defined in workflow yaml file for action's input, displayName and condition.  
+
+## Decision
+
+### Add `env` context in the runner
+
+Runner will create and populate the `env` context for every job execution using following logic:
+1. On job start, create `env` context with any environment variables in the job message, these are env defined in customer's YAML file's job/workflow level `env` section.
+2. Update `env` context when customer use `::set-env::` to set env at the runner level.
+
+The `env` context is only available in the runner, customer can't use the `env` context in any server evaluation part, just like the `runner` context
+
+Example yaml:
+```yaml
+
+env:
+  env1: 100
+jobs:
+  build:
+    env:
+      env2: 200
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          echo ${{ env.env1 }}
+        if: env.env2 == 200
+        name: ${{ env.env1 }}_${{ env.env2 }}
+```
+
+### Don't populate the `env` context with environment variables from runner machine. 
+
+With job container and container action, the `env` context may not have the right value customer want and will cause confusion.  
+Ex:
+```yaml
+build:
+  runs-on: ubuntu-latest   <- $USER=runner in hosted machine
+  container: ubuntu:16.04   <- $USER=root in container
+  steps:
+  - run: echo ${{env.USER}}    <- what should customer expect this output?  runner/root
+  - uses: docker://ubuntu:18.04
+    with:
+      args: echo ${{env.USER}}  <- what should customer expect this output? runner/root
+```