Update Dockerfile to pass tests

Also moves the source outside of the root directory, and changes
ENTRYPOINT to CMD to make it easier to enter the container and inspect
the test results.

Use -j8 to speed up builds, that should be fairly safe on most modern
machines.
diff --git a/Dockerfile b/Dockerfile
index 9cd4975..b4423be 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,6 +1,6 @@
-FROM ubuntu:18.04
+FROM ubuntu:20.04
 
-RUN apt-get update && apt-get install -y make git-core build-essential curl ninja-build python
+RUN apt-get update && apt-get install -y make git-core build-essential curl ninja-build python python3
 
 # Install Go
 RUN \
@@ -13,6 +13,7 @@
 ENV PATH $GOROOT/bin:$GOPATH/bin:$PATH
 
 # Copy project code.
-COPY . /
+COPY . /src
+WORKDIR /src
 
-ENTRYPOINT make test
+CMD make test -j8