fix: upgrade alpine version due to CVE-2020-28928 (#166)

diff --git a/all-in-one/apisix-dashboard/Dockerfile b/all-in-one/apisix-dashboard/Dockerfile
index 8f0937e..32d163e 100644
--- a/all-in-one/apisix-dashboard/Dockerfile
+++ b/all-in-one/apisix-dashboard/Dockerfile
@@ -30,7 +30,7 @@
 
 
 # Build etcd
-FROM alpine:3.11 AS etcd-stage
+FROM alpine:3.13 AS etcd-stage
 
 ARG ETCD_VERSION
 LABEL etcd_version="${ETCD_VERSION}"
@@ -90,7 +90,7 @@
     && yarn build
 
 # Finally combine all the resources into one image
-FROM alpine:3.11 AS last-stage
+FROM alpine:3.13 AS last-stage
 
 ARG ENABLE_PROXY
 
diff --git a/all-in-one/apisix/Dockerfile b/all-in-one/apisix/Dockerfile
index 0168065..97ea86f 100644
--- a/all-in-one/apisix/Dockerfile
+++ b/all-in-one/apisix/Dockerfile
@@ -29,7 +29,7 @@
     && apk del .builddeps build-base make unzip
 
 # Build etcd
-FROM alpine:3.11 AS etcd-stage
+FROM alpine:3.13 AS etcd-stage
 
 ARG ETCD_VERSION
 LABEL etcd_version="${ETCD_VERSION}"
@@ -41,7 +41,7 @@
     && ln -s etcd-${ETCD_VERSION}-linux-amd64 etcd
 
 # Finally combine all the resources into one image
-FROM alpine:3.11 AS last-stage
+FROM alpine:3.13 AS last-stage
 
 ARG ENABLE_PROXY
 # add runtime for Apache APISIX
diff --git a/alpine-dev/Dockerfile b/alpine-dev/Dockerfile
index 47e8a74..bfc3e18 100644
--- a/alpine-dev/Dockerfile
+++ b/alpine-dev/Dockerfile
@@ -20,7 +20,7 @@
     && mv /usr/local/apisix/deps/share/lua/5.1/apisix /usr/local/apisix \
     && apk del .builddeps build-base make unzip
 
-FROM alpine:3.11 AS last-stage
+FROM alpine:3.13 AS last-stage
 
 ARG ENABLE_PROXY
 # add runtime for Apache APISIX
diff --git a/alpine-local/Dockerfile b/alpine-local/Dockerfile
index ee8659f..cb812a5 100644
--- a/alpine-local/Dockerfile
+++ b/alpine-local/Dockerfile
@@ -20,7 +20,7 @@
     && mv ../apisix /usr/local/apisix \
     && apk del .builddeps build-base make unzip
 
-FROM alpine:3.11 AS last-stage
+FROM alpine:3.13 AS last-stage
 
 ARG ENABLE_PROXY
 # add runtime for Apache APISIX
diff --git a/alpine/Dockerfile b/alpine/Dockerfile
index ac37c6c..90dc95e 100644
--- a/alpine/Dockerfile
+++ b/alpine/Dockerfile
@@ -24,7 +24,7 @@
     && mv /usr/local/apisix/deps/share/lua/5.1/apisix /usr/local/apisix \
     && apk del .builddeps build-base make unzip
 
-FROM alpine:3.11 AS last-stage
+FROM alpine:3.13 AS last-stage
 
 ARG ENABLE_PROXY
 # add runtime for Apache APISIX