Reduce Docker image size

This is achieved with these techniques:
- Use Alpine as the base image, which is only ~5MB.
- Use docker's multi-stage build to keep only the result binaries in
  the final image.

The new image is ~15MB after this change.

Also updated Dockerfile to use the current checkout code instead of always
sync from the latest.

Also added a .dockerignore file to ignore temporary build artifacts.

Closes #535.

Change-Id: I3c90805ebba40295e69241214ed6d7adbde465b8

.dockerignore

@@ -0,0 +1,13 @@
+*.pyc
+*.sln
+*.VC.db
+*.vcxproj*
+*/.vs/*
+*~
+.DS_store
+.cproject
+.project
+.pydevproject
+.repo
+.settings
+/out*

Dockerfile

@@ -1,23 +1,31 @@
-FROM ubuntu:14.04
+FROM alpine:3.8 as builder
 
-ENV DEBIAN_FRONTEND noninteractive
+# Install packages needed for Shaka Packager.
+RUN apk add --no-cache bash build-base curl findutils git ninja python \
+                       bsd-compat-headers linux-headers libexecinfo-dev
 
-# update, and install basic packages
-RUN apt-get update
-RUN apt-get install -y \
-            build-essential \
-            curl \
-            git \
-            python
-
-# install depot_tools http://www.chromium.org/developers/how-tos/install-depot-tools
+# Install depot_tools.
 RUN git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
-ENV PATH /depot_tools:$PATH
+ENV PATH $PATH:/depot_tools
 
-# install shaka-packager
-RUN mkdir shaka_packager
+# Alpine uses musl which does not have mallinfo defined in malloc.h. Define the
+# structure to workaround a Chromium base bug.
+RUN sed -i \
+    '/malloc_usable_size/a \\nstruct mallinfo {\n  int arena;\n  int hblkhd;\n  int uordblks;\n};' \
+    /usr/include/malloc.h
+
+ENV GYP_DEFINES='clang=0 use_experimental_allocator_shim=0 use_allocator=none musl=1'
+
+# Build shaka-packager
 WORKDIR shaka_packager
-RUN gclient config https://www.github.com/google/shaka-packager.git --name=src
-RUN gclient sync --no-history
+RUN gclient config https://www.github.com/google/shaka-packager.git --name=src --unmanaged
+COPY . src
+RUN gclient sync
 RUN cd src && ninja -C out/Release
-ENV PATH /shaka_packager/src/out/Release:$PATH
+
+# Copy only result binaries to our final image.
+FROM alpine:3.8
+RUN apk add --no-cache libstdc++
+COPY --from=builder /shaka_packager/src/out/Release/packager \
+                    /shaka_packager/src/out/Release/mpd_generator \
+                    /usr/bin/