Upgrade go to 1.15.6 and alpine linux to 3.12.3 (#71)

* Upgrade alpine to 3.12.3

* Upgrade go to 1.15.6

* Update README.md
diff --git a/Dockerfile b/Dockerfile
index 8bc13c5..b2a549d 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -15,7 +15,7 @@
 # limitations under the License.
 #
 
-FROM alpine:3.12.1 as builder 
+FROM alpine:3.12.3 as builder 
 
 RUN apk add --no-cache --virtual .tools \
   bzip2 curl git automake libtool autoconf make sed file perl openrc openssl
@@ -70,8 +70,8 @@
   bash gcc musl-dev openssl go
 
 # Installing Golang https://github.com/CentOS/CentOS-Dockerfiles/blob/master/golang/centos7/Dockerfile
-RUN wget https://dl.google.com/go/go1.15.3.src.tar.gz \
-    && tar -C /usr/local -xzf go1.15.3.src.tar.gz && cd /usr/local/go/src/ && ./make.bash
+RUN wget https://dl.google.com/go/go1.15.6.src.tar.gz \
+    && tar -C /usr/local -xzf go1.15.6.src.tar.gz && cd /usr/local/go/src/ && ./make.bash
 ENV PATH=${PATH}:/usr/local/go/bin
 ENV GOPATH="/usr/local/go/bin"
 
@@ -107,7 +107,7 @@
 RUN chmod 755 records-config.sh
 RUN chmod 755 entry.sh
 
-FROM alpine:3.12.1
+FROM alpine:3.12.3
 
 COPY --from=builder /usr/local /usr/local
 
diff --git a/README.md b/README.md
index 82b7c62..e217d70 100644
--- a/README.md
+++ b/README.md
@@ -30,10 +30,10 @@
 - [Development](https://github.com/apache/trafficserver-ingress-controller/blob/master/docs/DEVELOPMENT.md)
 
 ## Versions of Software Used
-- Alpine 3.12.1
+- Alpine 3.12.3
 - Apache Traffic Server 8.1.1
 - LuaJIT 2.0.4 / Lua 5.1.4
-- Go 1.15.3
+- Go 1.15.6
 - Other Packages
   - luasocket 3.0rc1
   - redis-lua 2.0.4