docker: use shebangs to invoke generated scripts (#18295)

When generating scripts from templates, don't add a leading newline so
that their shebangs may be handled correctly.

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

---------

Co-authored-by: Quentin Gliech <quenting@element.io>

changelog.d/18295.docker

@@ -0,0 +1 @@
+When generating container scripts from templates, don't add a leading newline so that their shebangs may be handled correctly.

docker/Dockerfile-workers

@@ -74,4 +74,4 @@ FROM $FROM
     # Replace the healthcheck with one which checks *all* the workers. The script
     # is generated by configure_workers_and_start.py.
     HEALTHCHECK --start-period=5s --interval=15s --timeout=5s \
-        CMD /bin/sh /healthcheck.sh
+        CMD ["/healthcheck.sh"]

docker/complement/Dockerfile

@@ -58,4 +58,4 @@ ENTRYPOINT ["/start_for_complement.sh"]
 
 # Update the healthcheck to have a shorter check interval
 HEALTHCHECK --start-period=5s --interval=1s --timeout=1s \
-    CMD /bin/sh /healthcheck.sh
+    CMD ["/healthcheck.sh"]

docker/configure_workers_and_start.py

@@ -376,9 +376,11 @@ def convert(src: str, dst: str, **template_vars: object) -> None:
     #
     # We use append mode in case the files have already been written to by something else
     # (for instance, as part of the instructions in a dockerfile).
+    exists = os.path.isfile(dst)
     with open(dst, "a") as outfile:
         # In case the existing file doesn't end with a newline
-        outfile.write("\n")
+        if exists:
+            outfile.write("\n")
 
         outfile.write(rendered)
 
@@ -998,6 +1000,7 @@ def generate_worker_files(
         "/healthcheck.sh",
         healthcheck_urls=healthcheck_urls,
     )
+    os.chmod("/healthcheck.sh", 0o755)
 
     # Ensure the logging directory exists
     log_dir = data_dir + "/logs"