Various Docker image upgrades, read the entire commit message!

- Ubuntu 17.10 → 18.04.
- Python 3.6.3 → 3.6.6.
- Use `DEBIAN_FRONTEND=noninteractive` to prevent prompts during
  installation.
- Install `tzdata` in the base image as it's required by subimages.
- Correctly set maintainer in Dockerfile.

docker/1_base/Dockerfile

@@ -1,6 +1,10 @@
-FROM ubuntu:17.10
-MAINTAINER Francesco Siddi <francesco@blender.org>
+FROM ubuntu:18.04
+LABEL maintainer="Sybren A. Stüvel <sybren@blender.studio>"
 
-RUN apt-get update && apt-get install -qyy \
-    -o APT::Install-Recommends=false -o APT::Install-Suggests=false \
-    openssl ca-certificates
+RUN set -ex; \
+    apt-get update; \
+    DEBIAN_FRONTEND=noninteractive apt-get install \
+      -qyy -o APT::Install-Recommends=false -o APT::Install-Suggests=false \
+      tzdata openssl ca-certificates locales; \
+    locale-gen en_US.UTF-8 en_GB.UTF-8 nl_NL.UTF-8
+ENV LANG en_US.UTF-8