Need to update apk before installing glibc.

Dockerfile

@@ -8,11 +8,12 @@ RUN apk-install ca-certificates
 
 ENV TERRAFORM_VERSION 0.6.6
 
-RUN wget -q "https://circle-artifacts.com/gh/andyshinn/alpine-pkg-glibc/6/artifacts/0/home/ubuntu/alpine-pkg-glibc/packages/x86_64/glibc-2.21-r2.apk" && \
+RUN apk update && \
+    wget -q "https://circle-artifacts.com/gh/andyshinn/alpine-pkg-glibc/6/artifacts/0/home/ubuntu/alpine-pkg-glibc/packages/x86_64/glibc-2.21-r2.apk" && \
     apk add --allow-untrusted glibc-2.21-r2.apk && \
-    wget -q -O /terraform.zip https://releases.hashicorp.com/terraform/${TERRAFORM_VERSION}/terraform_${TERRAFORM_VERSION}_linux_amd64.zip && \
-    unzip /terraform.zip -d /bin && \
-    rm -rf /var/cache/apk/* glibc-2.21-r2.apk /terraform.zip
+    wget -q -O terraform.zip "https://releases.hashicorp.com/terraform/${TERRAFORM_VERSION}/terraform_${TERRAFORM_VERSION}_linux_amd64.zip" && \
+    unzip terraform.zip -d /bin && \
+    rm -rf /var/cache/apk/* glibc-2.21-r2.apk terraform.zip
 
 ADD drone-terraform /bin/
 ENTRYPOINT ["/bin/drone-terraform"]