ARROW-8330: [Documentation] The post release script generates the doc…

…umentation with a development version

Closes #6827 from kszucs/fix-docs-version

Authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>

dev/release/post-09-docs.sh

@@ -30,6 +30,7 @@ if [ "$#" -ne 1 ]; then
 fi
 
 version=$1
+release_tag="apache-arrow-${version}"
 branch_name=release-docs-${version}
 
 pushd "${ARROW_SITE_DIR}"
@@ -39,10 +40,15 @@ rm -rf docs/*
 popd
 
 pushd "${ARROW_DIR}"
+git checkout "${release_tag}"
+
 docker-compose build ubuntu-cpp
 docker-compose build ubuntu-python
 docker-compose build ubuntu-docs
-docker-compose run --rm -v "${ARROW_SITE_DIR}/docs:/build/docs" ubuntu-docs
+docker-compose run --rm \
+  -v "${ARROW_SITE_DIR}/docs:/build/docs" \
+  -e ARROW_DOCS_VERSION="${version}" \
+  ubuntu-docs
 popd
 
 pushd "${ARROW_SITE_DIR}"
@@ -53,4 +59,4 @@ popd
 
 echo "Success!"
 echo "Create a pull request:"
-echo "  ${github_url}/pull/new/${branch_name}"
+echo "  ${github_url}/pull/new/${branch_name}"