Upgrade CI to Ubuntu 20.04 LTS (Focal) (#1183)

# Why
On one hand Ubuntu18's free LTS cycle is ended, and on the other hand Ubuntu18 does not support nodejs greater than v18.
# What's changed.
1. update the Linux Docker image to buildpack-deps:focal
2. some compatibility changes
3. use the new recommended way to install Docker ([see here](https://docs.docker.com/engine/install/ubuntu/))

See #1173

.circleci/config.yml

diff --git a/.circleci/config.yml b/.circleci/config.yml
index cea7e0e..31f090e 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -4,9 +4,9 @@
   win: circleci/windows@5.0
 
 executors:
-  bionic:
+  ubuntu:
     docker:
-      - image: buildpack-deps:bionic
+      - image: buildpack-deps:focal
   mac:
     environment:
       EMSDK_NOTTY: "1"
@@ -50,22 +50,20 @@
 
 jobs:
   flake8:
-    executor: bionic
+    executor: ubuntu
     steps:
       - checkout
       - run:
           name: install pip
           command: |
             apt-get update -q
-            apt-get install -q -y python-pip python3-pip
-      - run: python2 -m pip install --upgrade pip
+            apt-get install -q -y python3-pip
       - run: python3 -m pip install --upgrade pip
-      - run: python2 -m pip install flake8==3.9.2
       - run: python3 -m pip install flake8==3.9.2
-      - run: python2 -m flake8 --show-source --statistics --extend-exclude=./scripts
-      - run: python3 -m flake8 --show-source --statistics
+      - run: python3 -m flake8 --show-source --statistics --extend-exclude=./scripts
+
   test-linux:
-    executor: bionic
+    executor: ubuntu
     environment:
       EMSDK_NOTTY: "1"
       # This is needed because the old gcc-7 that is installed on debian/bionic
@@ -174,12 +172,19 @@
             test/test_path_preservation.ps1
 
   build-docker-image-x64:
-    executor: bionic
+    executor: ubuntu
     steps:
       - checkout
       - run:
           name: install docker
-          command: apt-get update -q && apt-get install -q -y docker.io
+          command: |
+            apt-get update -q 
+            apt-get install -q -y ca-certificates curl gnupg lsb-release
+            mkdir -p /etc/apt/keyrings
+            curl -fsSL https://download.docker.com/linux/ubuntu/gpg | gpg --dearmor -o /etc/apt/keyrings/docker.gpg
+            echo "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.gpg] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable" | tee /etc/apt/sources.list.d/docker.list > /dev/null
+            apt-get update -q
+            apt-get install -q -y docker-ce docker-ce-cli containerd.io docker-compose-plugin
       - setup_remote_docker
       # Build the `latest` version of EMSDK as docker image
       - run:
@@ -190,12 +195,19 @@
           command: make -C ./docker version=latest test
 
   publish-docker-image-x64:
-    executor: bionic
+    executor: ubuntu
     steps:
       - checkout
       - run:
           name: install docker
-          command: apt-get update -q && apt-get install -q -y docker.io
+          command: |
+            apt-get update -q 
+            apt-get install -q -y ca-certificates curl gnupg lsb-release
+            mkdir -p /etc/apt/keyrings
+            curl -fsSL https://download.docker.com/linux/ubuntu/gpg | gpg --dearmor -o /etc/apt/keyrings/docker.gpg
+            echo "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.gpg] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable" | tee /etc/apt/sources.list.d/docker.list > /dev/null
+            apt-get update -q
+            apt-get install -q -y docker-ce docker-ce-cli containerd.io docker-compose-plugin
       - setup_remote_docker
       - run:
           name: build
@@ -226,7 +238,7 @@
             make -C ./docker version=${CIRCLE_TAG} alias=${CIRCLE_TAG}-arm64 only_alias=true push
 
   test-bazel-linux:
-    executor: bionic
+    executor: ubuntu
     steps:
       - checkout
       - run: apt-get install -q -y curl gnupg

README.md

diff --git a/README.md b/README.md
index 8e1c99b..9736aea 100644
--- a/README.md
+++ b/README.md
@@ -67,7 +67,7 @@
 - `python`: Version 2.7.0 or above.
 - `java`: For running closure compiler (optional)
 
-The emsdk pre-compiled binaries are built against Ubuntu/Xenial 16.04 LTS and
+The emsdk pre-compiled binaries are built against Ubuntu/Focal 20.04 LTS and
 therefore depend on system libraries compatible with versions of `glibc` and
 `libstdc++` present in that release.  If your linux distribution is very old
 you may not be able to use the pre-compiled binaries packages.