Build Plex on musl, from scratch

Plex now provide a first-party musl Plex build that works without any
external dependencies whatsoever. It's built with LLVM with many
compiler and linker optimisations enabled:
https://forums.plex.tv/t/plex-media-server-forum-preview-faster-and-smaller-builds-with-new-toolchain/699575

Changes for this release include:
- Drop curl, OpenSSL and zlib; they're no longer required. libcurl and
  libssl/libcrypto are provided by Plex anyway.
- Build `FROM spritsail/alpine` instead of `FROM debian` to ensure musl
  compatibility with all compiled binaries. Use `FROM scratch` for the
  resulting image. ld-musl is provided by Plex.
- Build busybox, su-exec and tini as they're no longer provided by the
  base image.
- Build binaries/libraries with standard hardening flags, including the
  popular -flto.

Signed-off-by: Joe Groocock <me@frebib.net>

.drone.yml

@@ -2,12 +2,6 @@
 kind: pipeline
 name: build-amd64
 
-trigger:
-  event:
-  - push
-  - pull_request
-  - tag
-
 platform:
   os: linux
   arch: amd64
@@ -24,7 +18,7 @@ steps:
   image: spritsail/docker-test
   settings:
     run: |
-      curl --version &&
+      busybox && \
       xmlstarlet --version
 
 - name: test
@@ -44,65 +38,15 @@ steps:
   settings:
     repo: spritsail/plex-media-server
     tags:
-    - latest
-    - "%label io.spritsail.version.plex | %remsuf [0-9a-f]+$ | %auto 2"
-    username: {from_secret: docker_username}
-    password: {from_secret: docker_password}
+    - musl
+    login: {from_secret: docker_login}
   when:
     branch:
-    - master
+    - musl
     event:
     - push
-
----
-kind: pipeline
-name: update-readme
-
-steps:
-- name: dockerhub-readme
-  pull: always
-  image: jlesage/drone-push-readme
-  settings:
-    username: {from_secret: docker_username}
-    password: {from_secret: docker_password}
-    repo: spritsail/plex-media-server
-  when:
-    branch:
-    - master
-    - pass
-    event:
-    - push
-
----
-kind: pipeline
-name: update-cron
-
-trigger:
-  event:
-  - cron
-
-platform:
-  os: linux
-  arch: amd64
-
-steps:
-- name: update
-  pull: always
-  image: spritsail/alpine
-  commands:
-  - apk add bash curl jq git
-  - ./update.sh
-
-- name: push
-  pull: always
-  image: appleboy/drone-git-push
-  settings:
-    branch: master
-    remote: git@github.com:spritsail/plex-media-server.git
-    ssh_key: {from_secret: git_ssh_key}
-
 ---
 kind: signature
-hmac: ffa33c4e8d4d71e4a1dcbc850859ebde679f6c574885d99df3ca9496e1db55b9
+hmac: 63bede647ba8e4f4f0a69983c22535fbda0afa811b60830c31edb0fbbf0b8b86
 
 ...