reuse devcontainer image in github actions

This is needed to run benchmarking tests in CI using the same `valgrind` binaries we install in the devcontainer.
Also moved the cache steps to a common action to avoid duplication.

.github/actions/cache/restore/action.yml

@@ -0,0 +1,18 @@
+name: "cache/restore"
+
+runs:
+  using: "composite"
+  steps:
+    - name: "Restore Cache"
+      uses: "actions/cache/restore@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9"
+      with:
+        # __SLANG_CI_CACHE_PATHS__ (keep in sync)
+        key: "cache-${{ github.ref_name }}-${{ hashFiles('Cargo.lock') }}-${{ hashFiles('package-lock.json') }}"
+        path: |
+          ~/.cache/hermit/
+          ~/.local/share/virtualenvs/
+          .hermit/
+        restore-keys: |
+          cache-${{ github.ref_name }}-
+          cache-main-
+          cache-

.github/actions/cache/save/action.yml

@@ -0,0 +1,18 @@
+name: "cache/save"
+
+runs:
+  using: "composite"
+  steps:
+    - name: "Repossess files from devcontainer user"
+      shell: "bash"
+      run: "sudo chown -R $USER:$USER $GITHUB_WORKSPACE"
+
+    - name: "Save Cache"
+      uses: "actions/cache/save@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9"
+      with:
+        # __SLANG_CI_CACHE_PATHS__ (keep in sync)
+        key: "cache-${{ github.ref_name }}-${{ hashFiles('Cargo.lock') }}-${{ hashFiles('package-lock.json') }}"
+        path: |
+          ~/.cache/hermit/
+          ~/.local/share/virtualenvs/
+          .hermit/

.github/actions/devcontainer/run/action.yml

@@ -0,0 +1,25 @@
+name: "devcontainer/run"
+
+inputs:
+  runCmd:
+    type: "string"
+    required: true
+
+  # TODO: this is a workaround for https://github.com/devcontainers/ci/pull/295
+  # Will remove and replace with 'inheritEnv' once the fix is released
+  env:
+    type: "string"
+    default: ""
+
+runs:
+  using: "composite"
+  steps:
+    - name: "Run in devcontainer"
+      uses: "devcontainers/ci@a56d055efecd725e8cfe370543b6071b79989cc8"
+      with:
+        configFile: ".devcontainer/devcontainer.json"
+        env: |
+          CI
+          ${{ inputs.env }}
+        push: "never"
+        runCmd: "${{ inputs.runCmd }}"