Adds SECURITY.md and scanning workflow (#223)

This adds SECURITY.md and a scanning workflow, using Trivy. In
particular, this clarifies what we use to scan for vulnerabilities
(Trivy, not anything else), and the only channel likely to be responded
to on a significant issue (zipkin-admin email, not advisories as people
ignored them).

This is the same approach as approved and merged in
openzipkin/zipkin-reporter-java#267

---------

Signed-off-by: Adrian Cole <[email protected]>

.github/workflows/security.yml

@@ -0,0 +1,50 @@
+---
+name: security
+
+# We don't scan documentation-only commits.
+on:  # yamllint disable-line rule:truthy
+  push:  # non-tagged pushes to master
+    branches:
+      - master
+    tags-ignore:
+      - '*'
+    paths-ignore:
+      - '**/*.md'
+  pull_request:  # pull requests targeted at the master branch.
+    branches:
+      - master
+    paths-ignore:
+      - '**/*.md'
+
+jobs:
+  security:
+    name: security
+    runs-on: ubuntu-22.04  # newest available distribution, aka jellyfish
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v4
+      - uses: actions/cache@v4
+        name: Cache Trivy Database
+        with:
+          path: .trivy
+          key: ${{ runner.os }}-trivy
+          restore-keys: ${{ runner.os }}-trivy
+      - name: Run Trivy vulnerability and secret scanner
+        uses: aquasecurity/trivy-action@master
+        id: trivy
+        with:
+          scan-type: 'fs'
+          scan-ref: '.'  # scan the entire repository
+          scanners: vuln,secret
+          exit-code: '1'
+          severity: HIGH,CRITICAL
+          output: trivy-report.md
+          cache-dir: .trivy
+      - name: Set Summary
+        shell: bash
+        if: ${{ failure() && steps.trivy.conclusion == 'failure' }}
+        # Add the Trivy report to the summary
+        #
+        # Note: This will cause a workflow error if trivy-report.md > the step
+        # limit 1MiB. If this was due to too many CVEs, consider fixing them ;)
+        run: cat trivy-report.md >> $GITHUB_STEP_SUMMARY

SECURITY.md

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+# OpenZipkin Security Process
+
+This document outlines the process for handling security concerns in OpenZipkin projects.
+
+Any vulnerability or misconfiguration detected in our [security workflow](.github/workflows/security.yml)
+should be addressed as a normal pull request.
+
+OpenZipkin is a volunteer community and does not have a dedicated security team. There may be
+periods where no volunteer is able to address a security concern. There is no SLA or warranty
+offered by volunteers. If you are a security researcher, please consider this before escalating.
+
+For security concerns that are sensitive or otherwise outside the scope of public issues, please
+contact [email protected].