Add Elixir CI to project

.github/workflows/elixir.yml

@@ -0,0 +1,76 @@
+name: Elixir CI
+
+on: [push, pull_request]
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  asdf:
+    name: ASDF
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+      # cache the ASDF directory, using the values from .tool-versions
+      - name: ASDF cache
+        uses: actions/cache@v3
+        with:
+          path: ~/.asdf
+          key: ${{ runner.os }}-asdf-v2-${{ hashFiles('.tool-versions') }}
+        id: asdf-cache
+      # only run `asdf install` if we didn't hit the cache
+      - uses: asdf-vm/actions/install@v2
+        if: steps.asdf-cache.outputs.cache-hit != 'true'
+      # only install Hex/Rebar if we didn't hit the cache
+      - if: steps.asdf-cache.outputs.cache-hit != 'true'
+        run: |
+          mix local.rebar --force
+          mix local.hex --force
+
+  build:
+    name: Build and test
+    runs-on: ubuntu-latest
+    needs: asdf
+    steps:
+      - uses: actions/checkout@v4
+      - name: ASDF cache
+        uses: actions/cache@v3
+        with:
+          path: ~/.asdf
+          key: ${{ runner.os }}-asdf-v2-${{ hashFiles('.tool-versions') }}
+        id: asdf-cache
+      - uses: mbta/actions/reshim-asdf@v1
+      # The asdf job should have prepared the cache. exit if it didn't for some reason
+      - run: exit 1
+        if: steps.asdf-cache.outputs.cache-hit != 'true'
+      - name: Restore dependencies cache
+        id: deps-cache
+        uses: actions/cache@v3
+        with:
+          path: deps
+          key: ${{ runner.os }}-mix-${{ hashFiles('**/mix.lock') }}
+          restore-keys: ${{ runner.os }}-mix-
+      - name: Install dependencies (if needed)
+        if: steps.deps-cache.outputs.cache-hit != 'true'
+        run: mix deps.get
+      - name: Compile (warnings as errors)
+        run: mix compile --force --warnings-as-errors
+      - name: Check formatting
+        run: mix format --check-formatted
+      - name: Credo
+        run: mix credo --strict
+      - name: Run tests
+        run: mix test --cover
+      - name: Save PR information
+        run: |
+          echo "${{ github.event.pull_request.number }}" > cover/PR_NUMBER
+          echo "${{ github.event.pull_request.head.sha }}" > cover/PR_SHA
+        if: github.event.pull_request
+      - name: Upload coverage artifact
+        uses: actions/upload-artifact@v2
+        with:
+          name: elixir-lcov
+          path: cover/
+      - uses: mbta/actions/dialyzer@v1