Fix 32-bit Promtail ARM docker builds from Drone

.drone/drone.jsonnet

@@ -153,7 +153,30 @@ local manifest(apps) = pipeline('manifest') {
     ],
   },
 ] + [
-  multiarch_image(arch)
+  multiarch_image(arch) + (
+    // When we're building Promtail for ARM, we want to use Dockerfile.arm32 to fix
+    // a problem with the published Drone image. See Dockerfile.arm32 for more
+    // information.
+    //
+    // This is really really hacky and a better more permanent solution will be to use
+    // buildkit.
+    if arch == 'arm'
+    then {
+      steps: [
+        step + (
+          if std.objectHas(step, 'settings') && step.settings.dockerfile == 'cmd/promtail/Dockerfile'
+          then {
+            settings+: {
+              dockerfile: 'cmd/promtail/Dockerfile.arm32',
+            },
+          }
+          else {}
+        )
+        for step in super.steps
+      ],
+    }
+    else {}
+  )
   for arch in archs
 ] + [
   fluentbit(),

.drone/drone.yml

@@ -368,7 +368,7 @@ steps:
   settings:
     build_args:
     - TOUCH_PROTOS=1
-    dockerfile: cmd/promtail/Dockerfile
+    dockerfile: cmd/promtail/Dockerfile.arm32
     dry_run: true
     password:
       from_secret: docker_password
@@ -424,7 +424,7 @@ steps:
   settings:
     build_args:
     - TOUCH_PROTOS=1
-    dockerfile: cmd/promtail/Dockerfile
+    dockerfile: cmd/promtail/Dockerfile.arm32
     password:
       from_secret: docker_password
     repo: grafana/promtail

cmd/promtail/Dockerfile.arm32

@@ -0,0 +1,30 @@
+FROM golang:1.13 as build
+# TOUCH_PROTOS signifies if we should touch the compiled proto files and thus not regenerate them.
+# This is helpful when file system timestamps can't be trusted with make
+ARG TOUCH_PROTOS
+COPY . /src/loki
+WORKDIR /src/loki
+RUN apt-get update && apt-get install -qy libsystemd-dev
+RUN make clean && (if [ "${TOUCH_PROTOS}" ]; then make touch-protos; fi) && make BUILD_IN_CONTAINER=false promtail
+
+# Promtail requires debian as the base image to support systemd journal reading
+FROM debian:stretch-slim
+# tzdata required for the timestamp stage to work
+RUN apt-get update && \
+  apt-get install -qy \
+  tzdata ca-certificates libsystemd-dev && \
+  rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
+COPY --from=build /src/loki/cmd/promtail/promtail /usr/bin/promtail
+COPY cmd/promtail/promtail-local-config.yaml /etc/promtail/local-config.yaml
+COPY cmd/promtail/promtail-docker-config.yaml /etc/promtail/docker-config.yaml
+
+# Drone CI builds arm32 images using armv8l rather than armv7l. Something in
+# our build process above causes ldconfig to be rerun and removes the armhf
+# library that debian:stretch-slim on ARM comes with. Symbolically linking to
+# ld-linux.so.3 fixes the problem and allows Promtail to start.
+#
+# This process isn't necessary when building on armv7l so we only do it if the
+# library was removed.
+RUN sh -c '[ ! -f /lib/ld-linux-armhf.so.3 ] && echo RE-LINKING LD-LINUX-ARMHF.SO.3 && ln -s /lib/ld-linux.so.3 /lib/ld-linux-armhf.so.3'
+
+ENTRYPOINT ["/usr/bin/promtail"]