Dockefile: format and use tagged version

[SKIP CI]

Dockerfile

@@ -1,27 +1,30 @@
-FROM golang:alpine AS binarybuilder
-# Install build deps
-RUN apk --no-cache --no-progress add --virtual build-deps build-base git linux-pam-dev
+FROM golang:alpine3.10 AS binarybuilder
+RUN apk --no-cache --no-progress add --virtual \
+  build-deps \
+  build-base \
+  git \
+  linux-pam-dev
+
 WORKDIR /go/src/github.com/gogs/gogs
 COPY . .
 RUN make build TAGS="sqlite cert pam"
 
-FROM alpine:latest
-# Install system utils & Gogs runtime dependencies
+FROM alpine:3.10
 ADD https://github.com/tianon/gosu/releases/download/1.10/gosu-amd64 /usr/sbin/gosu
 RUN chmod +x /usr/sbin/gosu \
   && echo http://dl-2.alpinelinux.org/alpine/edge/community/ >> /etc/apk/repositories \
   && apk --no-cache --no-progress add \
-    bash \
-    ca-certificates \
-    curl \
-    git \
-    linux-pam \
-    openssh \
-    s6 \
-    shadow \
-    socat \
-    tzdata \
-    rsync
+  bash \
+  ca-certificates \
+  curl \
+  git \
+  linux-pam \
+  openssh \
+  s6 \
+  shadow \
+  socat \
+  tzdata \
+  rsync
 
 ENV GOGS_CUSTOM /data/gogs