chore(outputs): add docs for outputs (#426)

content/usage/outputs.md

@@ -0,0 +1,88 @@
+---
+title: "Outputs"
+toc: true
+description: >
+  Learn how to configure outputs for steps.
+---
+
+{{% alert title="Tip:" color="info" %}}
+Outputs functionality is only available in the Docker runtime.
+{{% /alert %}}
+
+### What are outputs and why are they useful?
+
+While the workspace [is shared](/docs/usage/workspace) for all steps in a build, the environment is not. This is because changes to a container environment are not exported after the completion of a step.
+
+For example, take the following pipeline:
+
+```yaml
+version: "1"
+
+steps:
+    - name: parse image for testing
+      image: alpine:latest
+      commands:
+        # image to use is the first part of the source branch (inventory-updates -> my-organization/inventory:latest)
+        - image=$(echo ${VELA_PULL_REQUEST_SOURCE} | sed 's/[_.-].*//')
+
+    - name: test
+      image: my-organization/${image}:latest
+      pull: on_start
+      commands:
+        - go test ./...
+
+```
+
+This is not a valid pipeline configuration because `image` will not persist from the first step to the second. However, being able to dynamically update environment variables for substitution or for plugin parameters can be an important part of a CI build.
+
+This is where `outputs` can improve your pipeline configurations.
+
+Let's take the same pipeline but make it valid using outputs:
+
+```yaml
+version: "1"
+
+steps:
+    - name: parse image for testing
+      image: alpine:latest
+      commands:
+        # image to use is the first part of the source branch (inventory-updates -> my-organization/inventory:latest)
+        - echo "image=$(echo ${VELA_PULL_REQUEST_SOURCE} | sed 's/[_.-].*//')" > $VELA_OUTPUTS
+
+    - name: test
+      image: my-organization/${image}:latest
+      pull: on_start
+      commands:
+        - go test ./...
+```
+
+Writing to `$VELA_OUTPUTS` in environment file format ensures that those key-value pairs persist for the entirety of the build. 
+
+Not only is this more convenient than storing information in the shared workspace, it can also be safer. Vela secrets are masked in logs, but any sensitive value captured during a build has the potential to be leaked in logs if steps or plugins are not configured correctly.
+
+This can be remedied with masked outputs:
+
+```yaml
+version: "1"
+
+steps:
+    - name: capture API key for testing
+      image: alpine:latest
+      commands:
+        - apk add jq
+        - echo "API_KEY=$(curl http://my-test-endpoint/token | jq .token)" > $VELA_MASKED_OUTPUTS
+
+    - name: test
+      image: golang:latest
+      commands:
+        - echo $API_KEY # will print ***
+        - go test ./...
+```
+
+After the first step is complete, the logs will mask any mention of the `$API_KEY`. 
+
+### Limitations
+
+- Outputs can only be used as environment variables (`$VAR`) or substitution variables (`${VAR}`). Inline Go templating (`{{ .VAR }}`) is done at compile time and will not dynamically be evaluated.
+
+- Masking will only be applied to variables in `$VELA_MASKED_OUTPUTS` in steps that run _after_ the variable has been written. In other words, if `Step A` writes a token to `$VELA_MASKED_OUTPUTS`, `Step A` logs will _not_ mask the value. Only subsequent steps will have the masking applied.

themes/docsy/layouts/partials/head.html

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 {{- template "_internal/schema.html" . -}}
 {{- template "_internal/twitter_cards.html" . -}}
 {{ if eq (getenv "HUGO_ENV") "production" }}
-{{ template "_internal/google_analytics_async.html" . }}
+{{ template "_internal/google_analytics.html" . }}
 {{ end }}
 {{ partialCached "head-css.html" . "asdf" }}
 <script