Dockerfile: add nsswitch.conf, update go and dep

Dockerfile

@@ -1,8 +1,8 @@
-FROM golang:1.12-alpine
+FROM golang:1.13-alpine
 
 RUN apk update && apk add git
 RUN wget -O /usr/local/bin/dep \
-         https://github.com/golang/dep/releases/download/v0.5.0/dep-linux-amd64 \
+         https://github.com/golang/dep/releases/download/v0.5.4/dep-linux-amd64 \
  && chmod a+x /usr/local/bin/dep
 
 WORKDIR $GOPATH/src/github.com/ploxiln/oauth2_proxy/
@@ -14,7 +14,13 @@ RUN CGO_ENABLED=0 go build -ldflags="-s -w" -o /usr/local/bin/oauth2_proxy
 FROM busybox
 
 EXPOSE 4180
+
+# set up nsswitch.conf for Go's "netgo" implementation
+# https://github.com/golang/go/issues/35305
+RUN [ ! -e /etc/nsswitch.conf ] && echo 'hosts: files dns' > /etc/nsswitch.conf
+
 COPY --from=0  /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
 COPY --from=0  /usr/local/bin/oauth2_proxy        /usr/local/bin/
+
 USER www-data
 CMD ["oauth2_proxy"]