fix: Docker.dockerignore file can be huge. (#8023)

We run into some nutty error when dockerignore files get large (circleci
path):

```
#3 [internal] load metadata for docker.io/library/ubuntu:noble
#3 sha256:afb7fa05fafd7c9e9fe1ecefd45ae0531e51b736cd519c2fb8893340e4e08b59
#3 DONE 0.4s

#10 [internal] load build context
#10 sha256:8189bdeed2ad9e316ae51d8dddc8d96a01e5180ee99f9f37015f7c3dc8de7e41
#10 ERROR: rpc error: code = Internal desc = header list size to send violates the maximum size (1048896 bytes) set by server
```

This became the case with avm-transpiler as it's build context is the
root of the monorepo so it can gain access to the noir source code. The
autogenerated dockerignore file has to explicitly list every file in he
monorepo as a reverse match which eventually got so big something broke
internally to docker.

New script will not autogen the dockerignore file if its committed to
git. This PR includes a manually crafted one for avm-transpiler.

avm-transpiler/Dockerfile.dockerignore

@@ -0,0 +1,5 @@
+*
+!noir
+!avm-transpiler
+**/target
+**/node_modules

build-system/scripts/create_dockerignore

@@ -11,6 +11,12 @@ DOCKERFILE=$(query_manifest dockerfile $REPOSITORY)
 
 cd $BUILD_DIR
 DOCKERIGNOREFILE=$DOCKERFILE.dockerignore
+
+# If there is a dockerignore file committed to git, use it.
+if git ls-files --error-unmatch $DOCKERIGNOREFILE > /dev/null 2>&1; then
+  exit 0
+fi
+
 echo '*' > $DOCKERIGNOREFILE
 (git ls-files; git ls-files --others --exclude-standard) | sort -u | sed 's/^/!/' >> $DOCKERIGNOREFILE
-echo '**/Dockerfile*' >> $DOCKERIGNOREFILE
+echo '**/Dockerfile*' >> $DOCKERIGNOREFILE