Update ubuntu LTS to 20.04

code/docker/bitcoind/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04 AS bitcoind-base
+FROM ubuntu:20.04 AS bitcoind-base
 
 RUN apt update && apt install -yqq \
 	curl gosu jq bash-completion

code/docker/eclair/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:18.04 AS eclair-base
+FROM ubuntu:20.04 AS eclair-base
 
 RUN apt update && apt install -yqq \
 	curl gosu jq bash-completion

code/docker/lnd/Dockerfile

@@ -8,7 +8,7 @@ RUN go get -d github.com/lightningnetwork/lnd
 WORKDIR $GOPATH/src/github.com/lightningnetwork/lnd
 RUN make && make install
 
-FROM ubuntu:18.04 AS lnd-run
+FROM ubuntu:20.04 AS lnd-run
 
 RUN apt update && apt install -yqq \
 	curl gosu jq bash-completion

node_client.asciidoc

@@ -19,7 +19,7 @@ In this chapter you will learn how to set up each of the software packages for t
 
 ==== Using the command-line
 
-The examples in this chapter, and more broadly in most of this book, use a command-line terminal. That means that you type commands into a terminal and receive text responses. Furthermore, the examples are demonstrated on an operating system based on the Linux kernel and GNU software system, specifically the latest long-term stable release of Ubuntu (Ubuntu 18.04 LTS). The majority of the examples can be replicated on other operating systems such as Windows or Mac OS, with small modifications to the commands. The biggest difference between operating systems is the _package manager_ which installs the various software libraries and their pre-requisites. In the given examples, we will use +apt+, which is the package manager for Ubuntu. On Mac OS, a common package manager used for open source development is Homebrew (command +brew+) found at https://brew.sh.
+The examples in this chapter, and more broadly in most of this book, use a command-line terminal. That means that you type commands into a terminal and receive text responses. Furthermore, the examples are demonstrated on an operating system based on the Linux kernel and GNU software system, specifically the latest long-term stable release of Ubuntu (Ubuntu 20.04 LTS). The majority of the examples can be replicated on other operating systems such as Windows or Mac OS, with small modifications to the commands. The biggest difference between operating systems is the _package manager_ which installs the various software libraries and their pre-requisites. In the given examples, we will use +apt+, which is the package manager for Ubuntu. On Mac OS, a common package manager used for open source development is Homebrew (command +brew+) found at https://brew.sh.
 
 In most of the examples here, we will be building the software directly from the source code. While this can be quite challenging, it gives us the most power and control. You may choose to use docker containers, pre-compiled packages or other installation mechanisms instead if you get stuck!
 
@@ -211,7 +211,7 @@ Let's start by building and running the bitcoind container. First, we use the +d
 $ cd code/docker
 $ docker build -t lnbook/bitcoind bitcoind
 Sending build context to Docker daemon  12.29kB
-Step 1/25 : FROM ubuntu:18.04 AS bitcoind-base
+Step 1/25 : FROM ubuntu:20.04 AS bitcoind-base
  ---> c3c304cb4f22
 Step 2/25 : RUN apt update && apt install -yqq 	curl gosu jq bash-completion
 
@@ -457,10 +457,10 @@ The following additional packages will be installed:
 
  [...]
 
-Setting up libgcc-7-dev:amd64 (7.4.0-1ubuntu1~18.04.1) ...
-Setting up cpp-7 (7.4.0-1ubuntu1~18.04.1) ...
-Setting up libsodium-dev:amd64 (1.0.16-2) ...
-Setting up libstdc++-7-dev:amd64 (7.4.0-1ubuntu1~18.04.1) ...
+Setting up libsigsegv2:amd64 (2.12-2) ...
+Setting up libltdl-dev:amd64 (2.4.6-14) ...
+Setting up python2 (2.7.17-2ubuntu4) ...
+Setting up libsodium-dev:amd64 (1.0.18-1) ...
 
 [...]
 $
@@ -751,7 +751,7 @@ By this point, you are almost an expert in the basic operations of docker! In th
 $ cd code/docker
 $ docker build -t lnbook/eclair eclair
 Sending build context to Docker daemon  9.216kB
-Step 1/22 : FROM ubuntu:18.04 AS eclair-base
+Step 1/22 : FROM ubuntu:20.04 AS eclair-base
  ---> c3c304cb4f22
 Step 2/22 : RUN apt update && apt install -yqq 	curl gosu jq bash-completion
  ---> Using cache