Use COPY --link to improve layer caching

This commit will add `--link` to `COPY` commands in the Dockerfile. This
allows re-use of these layers, even when layers built previously change.

https://github.com/moby/buildkit/blob/HEAD/frontend/dockerfile/docs/syntax.md

Dockerfile

@@ -9,7 +9,7 @@ LABEL org.opencontainers.image.title="inboard"
 LABEL org.opencontainers.image.url="https://github.com/br3ndonland/inboard/pkgs/container/inboard"
 ARG LINUX_VERSION PIPX_VERSION=1.1.0 POETRY_VERSION=1.1.11
 ENV APP_MODULE=inboard.app.main_base:app LINUX_VERSION=$LINUX_VERSION PATH=/opt/pipx/bin:/app/.venv/bin:$PATH PIPX_BIN_DIR=/opt/pipx/bin PIPX_HOME=/opt/pipx/home PIPX_VERSION=$PIPX_VERSION POETRY_VERSION=$POETRY_VERSION PYTHONPATH=/app
-COPY poetry.lock poetry.toml pyproject.toml /app/
+COPY --link poetry.lock poetry.toml pyproject.toml /app/
 WORKDIR /app
 RUN <<HEREDOC
 . /etc/os-release
@@ -25,7 +25,7 @@ python -m pip install --no-cache-dir --upgrade pip "pipx==$PIPX_VERSION"
 pipx install "poetry==$POETRY_VERSION"
 poetry install --no-dev --no-interaction --no-root
 HEREDOC
-COPY inboard /app/inboard
+COPY --link inboard /app/inboard
 ENTRYPOINT ["python"]
 CMD ["-m", "inboard.start"]