specify the shell for RUN commands

[glennj]

This change doesn't affect the functionality of the docker build.

PR #22 was failing checks because it was seeing a "dockerfile parse error". This is even though the Dockerfile heredoc contained perfectly valid /bin/sh syntax.

Dockerfile:10
--------------------
   8 |     ENV DEBIAN_FRONTEND=noninteractive
   9 |     RUN << END_APT
  10 | >>>     set -e
  11 |         apt-get update -y
  12 |         apt-get install -y --no-install-recommends ca-certificates clang jq gawk locales curl
--------------------
ERROR: failed to build: failed to solve: dockerfile parse error on line 10: unknown instruction: set (did you mean user?)
Error: Process completed with exit code 1.

This change was suggested by Copilot, let's see if it helps.