Add action to filter on base branch.

This will be used to filter pull request events depending on which
branch they're targeting.

Author: lkysow

base-branch-filter/Dockerfile

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+FROM alpine:latest
+
+LABEL "com.github.actions.name"="base branch filter"
+LABEL "com.github.actions.description"="Filters pull request events based on their base branch."
+LABEL "com.github.actions.icon"="filter"
+LABEL "com.github.actions.color"="purple"
+
+LABEL "repository"="https://github.com/hashicorp/terraform-github-actions"
+LABEL "homepage"="http://github.com/hashicorp/terraform-github-actions"
+LABEL "maintainer"="HashiCorp Terraform Team <[email protected]>"
+
+RUN apk --no-cache add jq
+
+COPY entrypoint.sh /entrypoint.sh
+ENTRYPOINT ["/entrypoint.sh"]

base-branch-filter/README.md

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+# Base Branch Filter
+Filters pull request events depending on the base branch
+The base branch is the branch that the pull request will be merged into.
+
+To use, set `args` to a regular expression that
+will be matched against the destination branch.
+
+Note: This action only works on pull request events.
+
+## Example
+Filter on pull requests that have been merged into `master`:
+```hcl
+workflow "example" {
+  resolves = "base-branch-filter"
+  # Must be used on pull_request events.
+  on = "pull_request"
+}
+
+# First we use another filter to filter to only merged events.
+action "merged-prs-filter" {
+  uses = "actions/bin/filter@master"
+  args = "merged true"
+}
+
+# Then we use this filter to ensure the branch matches "master".
+action "base-branch-filter" {
+  uses = "hashicorp/terraform-github-actions/base-branc-filter@master"
+  # We set args to our regex.
+  args = "^master$"
+  needs = "merged-prs-filter"
+}
+```

base-branch-filter/entrypoint.sh

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+#!/bin/sh
+
+set -e
+
+regex="$*"
+base_branch=$(jq -r .pull_request.base.ref "$GITHUB_EVENT_PATH")
+
+if "$actual" | grep -q "$regex"; then
+  echo "base branch \"$base_branch\" does not match \"$regex\""
+  exit 78
+fi