Stop shipping 32 GB to the daemon to build an image that reads none of it

Neither Dockerfile has a COPY or an ADD. Both are environment images —
CI checks the repo out inside the container, local use bind-mounts it —
so every byte of the build context was uploaded and then ignored. Here
that is 32 GB, 30 GB of it target/, before the first apt line runs.

The registry has never held either image, so this had never actually
been paid: the workflows were committed and the push never happened.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,11 @@
+# Both Dockerfiles are *environment* images: neither has a single COPY or ADD,
+# because CI checks the repo out inside the container at run time and local use
+# bind-mounts it. So the build context is deliberately empty.
+#
+# Without this file `docker build .` uploads the whole tree to the daemon —
+# 32 GB here, 30 GB of it target/ — to build an image that reads none of it.
+#
+# If a Dockerfile ever does need a file from the tree, un-ignore it explicitly:
+#   *
+#   !scripts/needed-thing.sh
+*