Replace Gzip with PGzip (#266)

* Replace Gzip with optimized PGzip. Add concurrency option.

* Add shortened timeout for 'dc down' too.

* Add NaturalNumberZero to allow zero.

* Add test for concurrency=0

* Rename to GZIP_PARALLELISM

* Fix block size. Fix compression level. Fix CI.

* Refactor compression writer fetching. Renamed WholeNumber

test/pgzip/run.sh

@@ -0,0 +1,42 @@
+#!/bin/sh
+
+set -e
+
+cd $(dirname $0)
+. ../util.sh
+current_test=$(basename $(pwd))
+
+docker network create test_network
+docker volume create app_data
+
+LOCAL_DIR=$(mktemp -d)
+
+docker run -d -q \
+  --name offen \
+  --network test_network \
+  -v app_data:/var/opt/offen/ \
+  offen/offen:latest
+
+sleep 5
+
+docker run --rm -q \
+  --network test_network \
+  -v app_data:/backup/app_data \
+  -v $LOCAL_DIR:/archive \
+  -v /var/run/docker.sock:/var/run/docker.sock \
+  --env BACKUP_COMPRESSION=gz \
+  --env GZIP_PARALLELISM=0 \
+  --env BACKUP_FILENAME='test.{{ .Extension }}' \
+  --entrypoint backup \
+  offen/docker-volume-backup:${TEST_VERSION:-canary}
+
+tmp_dir=$(mktemp -d)
+tar -xvf "$LOCAL_DIR/test.tar.gz" -C $tmp_dir
+if [ ! -f "$tmp_dir/backup/app_data/offen.db" ]; then
+  fail "Could not find expected file in untared archive."
+fi
+pass "Found relevant files in untared local backup."
+
+# This test does not stop containers during backup. This is happening on
+# purpose in order to cover this setup as well.
+expect_running_containers "1"