Adding ability to serve TLS directly from Mattermost server (#4119)

vendor/github.com/xenolf/lego/Dockerfile

@@ -0,0 +1,14 @@
+FROM alpine:3.4
+
+ENV GOPATH /go
+
+RUN apk update && apk add ca-certificates go git && \
+    rm -rf /var/cache/apk/* && \
+    go get -u github.com/xenolf/lego && \
+    cd /go/src/github.com/xenolf/lego && \
+    go build -o /usr/bin/lego . && \
+    apk del ca-certificates go git && \
+    rm -rf /var/cache/apk/* && \
+    rm -rf /go
+
+ENTRYPOINT [ "/usr/bin/lego" ]