remove PEM support

Author: sfotony

Dockerfile

@@ -1,6 +1,6 @@
 FROM golang:1.15.3-alpine AS build-env
 RUN  apk add --no-cache git make ca-certificates
-LABEL maintainer="@sfotony (github.com/sfotony)"
+LABEL maintainer="@sfotony (github.com/sfotony), based on project by @amimof (github.com/amimof)""
 COPY . /go/src/github.com/sfotony/node-cert-exporter
 WORKDIR /go/src/github.com/sfotony/node-cert-exporter
 RUN make