create_template.sh produces null templateID for multi-line Dockerfiles

**Reproduction**

Calling `./packages/create_template.sh --docker-file ` with any Dockerfile
that has more than one line (i.e., almost all real Dockerfiles) results in:

- The API returning an empty body
- Downstream `jq -r '.templateID'` producing the literal string `"null"`
- ECR push attempts landing at `e2bdev/base/null:null`
- Subsequent build polling spinning forever on status `null`

**Minimal reproducer**

Run ./packages/create_template.sh --docker-file Dockerfile against any
deployed stack. Observe the Response: line is empty/malformed.

**Root cause**

In packages/create_template.sh:78-89, the curl -d body is built via
naive bash JSON interpolation:

-d "{
\"readyCmd\": \"$READY_COMMAND\",
...
\"dockerfile\": \"$DOCKERFILE\",
...
}"
When $DOCKERFILE contains literal newlines (any multi-line Dockerfile),
the resulting body is invalid JSON: newlines are unescaped inside a JSON
string, and any " inside the Dockerfile (e.g., RUN echo "hello")
prematurely terminates the dockerfile field.

**Fix proposed**

Replace the bash interpolation with jq -n --arg dockerfile "$DOCKERFILE" ...
which handles all valid Dockerfile content correctly (newlines, quotes,
backslashes). Falls back to Python json.dumps() when jq is unavailable
(though jq is already a hard dependency elsewhere in the script, so the
fallback is mostly belt-and-suspenders).

I'd like to send a PR for this — happy to make any structural adjustments
the maintainers prefer.