fix: Do not delete release image that is just built

If building a release-image results in an image that already exists, then that image may not the most recent one. Which causes the image that was just built to be immediately untagged:

```
$ ./bootstrap.sh
--- release-image build ---
Cache download and extraction of release-image-base-23b1b4c058edacf1.zst complete in 17s.
The image aztecprotocol/release-image-base:latest already exists, renaming the old one with ID sha256:eab31bb60893aeba8bc580463c3e1e871004fa8d261075fb214204e34897b2e6 to empty string
Loaded image: aztecprotocol/release-image-base:latest
[+] Building 2.4s (9/9) FINISHED                                                                                     docker:default
 => [internal] load build definition from Dockerfile                                                                           0.0s
 => => transferring dockerfile: 2.10kB                                                                                         0.0s
 => [internal] load metadata for docker.io/aztecprotocol/release-image-base:latest                                             0.0s
 => [internal] load build context                                                                                              2.3s
 => => transferring context: 454.61kB                                                                                          2.3s
 => [1/5] FROM docker.io/aztecprotocol/release-image-base:latest                                                               0.0s
 => CACHED [2/5] COPY . /usr/src                                                                                               0.0s
 => CACHED [3/5] RUN mkdir -p /usr/src/bb /usr/src/acvm && chmod 777 /usr/src/bb /usr/src/acvm                                 0.0s
 => CACHED [4/5] WORKDIR /usr/src/yarn-project                                                                                 0.0s
 => CACHED [5/5] RUN echo '{".": "'1fd7c0847f495ac2e9ffd701e7958d5893b6f897'"}' > /usr/src/.release-please-manifest.json       0.0s
 => exporting to image                                                                                                         0.0s
 => => exporting layers                                                                                                        0.0s
 => => writing image sha256:cdcb8e01b2139ca7cdcee3e195b6816fa924902b83acae79b6c26d0631efd462                                   0.0s
 => => naming to docker.io/aztecprotocol/aztec:1fd7c0847f495ac2e9ffd701e7958d5893b6f897                                        0.0s
Untagged: aztecprotocol/aztec:1fd7c0847f495ac2e9ffd701e7958d5893b6f897
Untagged: aztecprotocol/aztec:latest
Deleted: sha256:cdcb8e01b2139ca7cdcee3e195b6816fa924902b83acae79b6c26d0631efd462
```

Co-authored-by: Santiago Palladino <santiago@aztec-labs.com>
Co-authored-by: ludamad <adam.domurad@gmail.com>

Author: spalladino

release-image/bootstrap.sh

@@ -16,11 +16,15 @@ function build_image {
     # Otherwise, use the commit hash as the version.
     local version=$(git rev-parse HEAD)
   fi
+  local previous_ids=$(docker images aztecprotocol/aztec --format "{{.ID}}" | uniq)
   docker build -f release-image/Dockerfile --build-arg VERSION=$version -t aztecprotocol/aztec:$(git rev-parse HEAD) .
   docker tag aztecprotocol/aztec:$(git rev-parse HEAD) aztecprotocol/aztec:latest
 
-  # Remove all but the most recent image.
-  docker images aztecprotocol/aztec --format "{{.ID}}" | uniq | tail -n +2 | xargs -r docker rmi -f
+  # If we actually built a new image (not from cache), remove all but the just-built image.
+  local new_ids=$(docker images aztecprotocol/aztec --format "{{.ID}}" | uniq)
+  if [ "$previous_ids" != "$new_ids" ]; then
+    echo "$previous_ids" | xargs -r docker rmi -f
+  fi
 }
 export -f build_image