Reasonable memory restrictions when running local minio s3 plugin test

Set 2g memory limit to local Minio docker.
Set s3 configuration chunksize and concurrency.

S3 plugin upload and download chunksize defaults to 500MB and concurrent
requests defaults to 6. This means that each s3 plugin upload call will
reserve a buffer size of 500MB * 6 = 3G.

This is observable during testing in the s3 plugin logs.
```
Uploading file [backup_file] with chunksize 524288000 and concurrency 6
```

On a demo cluster of 3 segments
this means that during backup or restore our plugin will try to reserve
a total of 3G * 3 segments = 9G. This way out of line for local minio
testing which only tries to upload a few bytes. If a system doesn't have
this kind of free memory on hand it will error out with the following
panic.

```
fatal error: runtime: out of memory

runtime stack:
...

goroutine 1 [running]:
runtime.systemstack_switch()
	/usr/local/go/src/runtime/asm_amd64.s:474 +0x8 fp=0xc00013f158 sp=0xc00013f148 pc=0x46a788
runtime.(*mheap).alloc(0x0?, 0x3?, 0xb8?)
	/usr/local/go/src/runtime/mheap.go:962 +0x5b fp=0xc00013f1a0 sp=0xc00013f158 pc=0x42cb3b
runtime.(*mcache).allocLarge(0xc000024540?, 0xbb800000, 0x58?)
	/usr/local/go/src/runtime/mcache.go:234 +0x85 fp=0xc00013f1e8 sp=0xc00013f1a0 pc=0x41ab85
runtime.mallocgc(0xbb800000, 0xa39600, 0x1)
	/usr/local/go/src/runtime/malloc.go:1123 +0x4f6 fp=0xc00013f250 sp=0xc00013f1e8 pc=0x411d96
runtime.makeslice(0xb5aa93?, 0x36?, 0xc00013f3a0?)
	/usr/local/go/src/runtime/slice.go:103 +0x49 fp=0xc00013f278 sp=0xc00013f250 pc=0x451d69
bufio.NewReaderSize(...)
	/usr/local/go/src/bufio/bufio.go:57
github.com/greenplum-db/gpbackup-s3-plugin/s3plugin.uploadFile(0xc0000305d0?, 0xc0002f1c20, {0xc0002e6460, 0x4a}, 0xc00003c148)
...
```

Makefile

@@ -141,7 +141,7 @@ info-report:
 test-s3-local: build install
 	${PWD}/plugins/generate_minio_config.sh
 	mkdir -p /tmp/minio/gpbackup-s3-test
-	docker run -d --name s3-minio -p 9000:9000 -p 9001:9001 -v /tmp/minio:/data/minio quay.io/minio/minio server /data/minio --console-address ":9001"
+	docker run -d --name s3-minio --memory="2g" -p 9000:9000 -p 9001:9001 -v /tmp/minio:/data/minio quay.io/minio/minio server /data/minio --console-address ":9001"
 	sleep 2 # Wait for minio server to start up
 	${PWD}/plugins/plugin_test.sh $(BIN_DIR)/gpbackup_s3_plugin /tmp/minio_config.yaml
 	docker stop s3-minio

plugins/generate_minio_config.sh

@@ -8,4 +8,8 @@ options:
   aws_secret_access_key: minioadmin
   bucket: gpbackup-s3-test
   folder: test/backup
+  backup_max_concurrent_requests: 2
+  backup_multipart_chunksize: 5MB
+  restore_max_concurrent_requests: 2
+  restore_multipart_chunksize: 5MB
 MINIO_CONFIG