Added Dockerfile and fixed test dependency

Dockerfile

@@ -0,0 +1,9 @@
+FROM swift:5.1 as builder
+
+RUN apt-get -qq update && apt-get install -y \
+  libssl-dev zlib1g-dev \
+  && rm -r /var/lib/apt/lists/*
+
+WORKDIR /aws-signer
+COPY . .
+RUN swift test

Package.swift

@@ -16,7 +16,7 @@ let package = Package(
     targets: [
         .target(name: "AWSSigner", dependencies: ["NIOHTTP1"]),
         .target(name: "HTTPClientAWSSigner", dependencies: ["AWSSigner", "AsyncHTTPClient"]),
-        .testTarget(name: "AWSSignerTests", dependencies: ["AWSSigner"]),
+        .testTarget(name: "AWSSignerTests", dependencies: ["AWSSigner", "HTTPClientAWSSigner"]),
     ]
 )