Add information about how to run lambda locally

Makefile

@@ -53,6 +53,12 @@ target/lambda/release/email_lambda: Cargo.lock $(SHARED_SRC) $(LAMBDA_SRC)
 		-v ${HOME}/.cargo/git:/root/.cargo/git \
 		softprops/lambda-rust:${V_LAMBDA_RUST}
 
+target/lambda/runnable/fn:
+	mkdir -p target/lambda/runnable/fn
+
+target/lambda/runnable/fn/bootstrap: target/lambda/runnable/fn target/lambda/release/email_lambda
+	cp target/lambda/release/email_lambda $@
+
 # Build a zip archive which can be uploaded to AWS lambda
 email_lambda.zip: target/lambda/release/email_lambda
 	cp ./target/lambda/release/email_lambda ./bootstrap \

README.md

@@ -67,6 +67,30 @@ have:
 - A DynamoDB table name
 - AWS credentials in the environment that can access both
 
+### Localstack
+
+An emulation of the DynamoDB and SQS resources can be created with Docker and
+the [localstack][localstack] image.
+[`docker-compose.yml`](./docker-compose.yml) is configured to start localstack
+with these services but starting the image does not configure DynamoDB tables
+or SQS queues.
+
+```shell
+# Create an SQS queue called emails_local
+aws --endpoint-url=http://localhost:4566 \
+  sqs create-queue \
+  --queue-name=emails_local
+# Create a DynamoDB table called emails_local
+aws --endpoint-url=http://localhost:4566 \
+  dynamodb create-table \
+  --attribute-definitions=AttributeName=EmailId,AttributeType=S \
+  --table-name=emails_local \
+  --key-schema=AttributeName=EmailId,KeyType=HASH \
+  --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5
+```
+
+[localstack]: https://github.com/localstack/localstack
+
 ### Test
 
 ```shell
@@ -108,6 +132,21 @@ aws lambda update-function-code --function-name <function_name> \
   --zip-file fileb://./email_lambda.zip
 ```
 
+#### Run Lambda with Localstack
+
+```shell
+make target/lambda/runnable/fn/bootstrap
+echo '{"Records":[]}' \
+  | docker run \
+      -i \
+      -e DOCKER_LAMBDA_USE_STDIN=1 \
+      -e DYNAMO_TABLE="emails_local" \
+      -e QUEUE_URL="http://localhost:4566/000000000000/emails_local" \
+      --rm \
+      -v "${PWD}/target/lambda/runnable/fn":/var/task \
+      lambci/lambda:provided
+```
+
 ### Deploy with AWS CDK
 
 List the stacks available for deployment