Fix a Dockerfile break I shipped and did not test

The deploy failed:
  dockerfile parse error on line 50: unknown instruction: BOQ_BUILD_SHA=dev

I appended BOQ_BUILD_SHA=dev after an ENV block that had no trailing
continuation, so it became a standalone Dockerfile instruction. It was also
wrong-headed: it would have overridden the real SHA baked in by ARG with the
literal string 'dev'. The ARG/ENV pair further up already does the job.

This one is on me. I changed a Dockerfile, pushed it, and told the user to
redeploy, without ever building the image. The earlier deploy was fine
precisely because that image HAD been built and run first.

Verified before pushing this time:
  docker build --build-arg BUILD_SHA=51e4600  -> ok
  container reports BOQ_BUILD_SHA=51e4600
  /_stcore/health -> ok
  Central Towers converts inside the image: 192 written, 167 correct,
    23 unplaced, 0 formulas clobbered, gate not blocked

Dockerfile

@@ -43,7 +43,6 @@ ENV HOME=/home/megainfra \
     BOQ_PROFILES=/data/profiles \
     STREAMLIT_BROWSER_GATHER_USAGE_STATS=false \
     MPLCONFIGDIR=/tmp/mpl
-    BOQ_BUILD_SHA=dev
 
 EXPOSE 8501