Upgrade all docker containers to 18.04

ci/docker/aarch64-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     gcc libc6-dev ca-certificates \

ci/docker/arm-unknown-linux-gnueabi/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     gcc libc6-dev ca-certificates \

ci/docker/arm-unknown-linux-gnueabihf/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     gcc libc6-dev ca-certificates \

ci/docker/armv7-unknown-linux-gnueabihf/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     gcc libc6-dev ca-certificates \

ci/docker/i586-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     gcc-multilib libc6-dev ca-certificates

ci/docker/i686-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     gcc-multilib libc6-dev ca-certificates

ci/docker/mips-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \

ci/docker/mips64-unknown-linux-gnuabi64/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     ca-certificates \

ci/docker/mips64el-unknown-linux-gnuabi64/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     ca-certificates \

ci/docker/mipsel-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \

ci/docker/powerpc-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \

ci/docker/powerpc64-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \

ci/docker/powerpc64le-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \

ci/docker/thumbv6m-linux-eabi/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     ca-certificates curl gcc gcc-arm-none-eabi libc6-dev libcurl4-openssl-dev libssh2-1 libnewlib-dev qemu-user-static

ci/docker/thumbv7em-linux-eabi/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     ca-certificates curl gcc gcc-arm-none-eabi libc6-dev libcurl4-openssl-dev libssh2-1 libnewlib-dev qemu-user-static

ci/docker/thumbv7em-linux-eabihf/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     ca-certificates curl gcc gcc-arm-none-eabi libc6-dev libcurl4-openssl-dev libssh2-1 libnewlib-dev qemu-user-static

ci/docker/thumbv7m-linux-eabi/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     ca-certificates curl gcc gcc-arm-none-eabi libc6-dev libcurl4-openssl-dev libssh2-1 libnewlib-dev qemu-user-static

ci/docker/x86_64-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
     gcc libc6-dev ca-certificates