Generate a tag derived from the inputs to a Docker image, without needing to build the image itself.

docker-content-tag.sh

#!/bin/bash

cat <<'EOF' >.dockerfile.content-tag
FROM busybox:1
WORKDIR /src/
ENTRYPOINT ["/bin/sh", "-c", "find . -type f \\! -path ./.dockerfile.content-tag -exec sha256sum -b {} +"]
COPY . /src/
EOF

iid=$( docker image build -q -f .dockerfile.content-tag . )

docker container run --rm "${iid}" |
  LC_ALL=C sort -k1.67 | sha256sum -b | cut -c-7

rm .dockerfile.content-tag
docker image rm "${iid}"