chore(ci) cache httpbin docker image

CI has been failing rather frequently due to Docker registry returning
`502 Bad Gateway` when the `setup-httpbin-server` action attempts to
pull httpbin's docker image.

In fact, this image has been pulled by 16 jobs (Unit and Valgrind) for every GHA
workflow run. This might explain why we eventually fail to pull it.

Caching it should greatly reduce the number of times httpbin image is
actually pulled, which should prevent us from being rate limited.

.github/actions/load-docker-image/action.yml

@@ -0,0 +1,52 @@
+name: load-docker-image
+decription: |
+  Pull a docker image and cache it.
+
+inputs:
+  image:
+    required: true
+    type: string
+  cache_dir:
+    required: false
+    type: string
+    default: "$GITHUB_WORKSPACE/.docker-images-cache"
+
+runs:
+  using: 'composite'
+  steps:
+    - name: Create cache directory
+      shell: bash
+      run: mkdir -p ${{ inputs.cache_dir }}
+
+    - name: Setup cache
+      uses: actions/cache@v4
+      with:
+          path: ${{ inputs.cache_dir }}
+          key: pulled-docker-images
+
+    - name: Setup cache path
+      id: cache_setup
+      shell: bash
+      run: |
+        filename="$(echo ${{ inputs.image }} | sed 's/\//_/g').tar"
+        cache_path="${{ inputs.cache_dir }}/$filename"
+
+        if [ -f "$cache_path" ]; then
+          echo "cache_hit=true" >> $GITHUB_OUTPUT
+        else
+          echo "cache_hit=false" >> $GITHUB_OUTPUT
+        fi
+
+        echo "cache_path=$cache_path" >> $GITHUB_OUTPUT
+
+    - name: Pull Docker image
+      if: steps.cache_setup.outputs.cache_hit == 'false'
+      shell: bash
+      run: |
+        docker pull ${{ inputs.image }}
+        docker save -o ${{ steps.cache_setup.outputs.cache_path }} ${{ inputs.image }}
+
+    - name: Load Docker image from cache
+      if: steps.cache_setup.outputs.cache-hit == 'true'
+      shell: bash
+      run: docker load -i ${{ steps.cache_setup.outputs.cache_path }}

.github/actions/setup-httpbin-server/action.yml

@@ -36,12 +36,14 @@ runs:
 
         sudo apt-get update
         sudo apt-get install -y dnsmasq
+
     - name: 'Setup deps - macOS'
       if: ${{ contains(inputs.os, 'macos') }}
       shell: bash
       run: |
         brew install dnsmasq docker colima
         colima start --network-address
+
     - name: Setup Docker image tag
       id: setup
       shell: bash
@@ -54,16 +56,19 @@ runs:
             echo "push=false" >> $GITHUB_OUTPUT
             echo "tag=wasmx-ci-httpbin-proxy:latest" >> $GITHUB_OUTPUT
           fi
+
     - name: Login to GitHub Container Registry
       if: ${{ steps.setup.outputs.push == 'true' }}
       uses: docker/login-action@v3
       with:
         registry: ghcr.io
         username: ${{ inputs.ghcr_username }}
         password: ${{ inputs.ghcr_password }}
+
     - name: Setup Docker Buildx
       if: ${{ !env.ACT }}
       uses: docker/setup-buildx-action@v3
+
     - name: Build httpbin-proxy image
       uses: docker/build-push-action@v5
       with:
@@ -74,6 +79,13 @@ runs:
         context: .
         cache-from: type=gha
         cache-to: type=gha,mode=max
+
+    - name: Load httpbin docker image
+      if: ${{ !env.ACT }}
+      uses: ./.github/actions/load-docker-image
+      with:
+        image: "kennethreitz/httpbin"
+
     - name: Start dnsmasq
       shell: bash
       run: |
@@ -83,6 +95,7 @@ runs:
                 --server=${{ inputs.upstream_dns_server }} \
                 --address=/httpbin.org/127.0.0.1 \
                 --address=/example.com/127.0.0.1
+
     - name: Start httpbin proxy + server
       shell: bash
       run: |