Docker build improvements (#8218)

Fixes https://github.com/certbot/certbot/issues/8208.
Fixes https://github.com/certbot/certbot/issues/8198.

In addition to those two linked issues, this PR:

* Splits both the build and deploy steps based on architecture for performance. The Docker builds should no longer be the bottleneck in any stage of the pipeline.
* Skips building Docker images for ARM on `test-` branches like [we do for snaps](https://github.com/certbot/certbot/blob/e8a232297d6faedb3b0bffc197a2e98c2e59edcb/.azure-pipelines/templates/jobs/packaging-jobs.yml#L67-L71). I initially didn't want to do this, but the ARM builds take ~18 minutes which is significantly longer than any other job currently running on our `test-` branches.

You can see tests running on my fork at:

* [Release pipeline](https://dev.azure.com/bmw0523/bmw/_build/results?buildId=387&view=results)
* [Test pipeline](https://dev.azure.com/bmw0523/bmw/_build/results?buildId=388&view=results)
* [Nightly pipeline](https://dev.azure.com/bmw0523/bmw/_build/results?buildId=390&view=results)

* update script intro

* update readme

* ParseRequestedArch

* build all arch in Azure

* Build docker images during testing/packaging.

* require global variable?

* Error if TAG_BASE is empty.

* prepare build job

* change variable syntax

* Update deploy stage.

* remove old dockerTag param

* add displayName

* fix docker images command

* split docker_build by arch

* Allow deploying a subset of architectures.

* deploy in parallel

* Skip ARM builds on test- branches.

* fix spacing

tools/docker/deploy.sh

@@ -4,8 +4,9 @@ IFS=$'\n\t'
 
 # This script deploys new versions of Certbot and Certbot plugin docker images.
 
-# Usage: ./deploy.sh [TAG]
-#   with [TAG] corresponding the base of the tag to give the Docker images.
+# Usage: ./deploy.sh [TAG] [all|amd64|arm32v6|arm64v8]
+#   with the [TAG] value corresponding the base of the tag to give the Docker
+#   images and the 2nd value being the architecture to build snaps for.
 #   Values should be something like `v0.34.0` or `nightly`. The given value is
 #   only the base of the tag because the things like the CPU architecture are
 #   also added to the full tag.
@@ -13,12 +14,18 @@ IFS=$'\n\t'
 WORK_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null && pwd )"
 
 TAG_BASE="$1"  # Eg. v0.35.0 or nightly
+if [ -z "$TAG_BASE" ]; then
+    echo "We cannot tag Docker images with an empty string!" >&2
+    exit 1
+fi
 source "$WORK_DIR/lib/common"
+ParseRequestedArch "${2}"
 
-# Creates and pushes all Docker images aliases for all architectures.
-# If the value of the global variable TAG_BASE is a version tag such as
-# v0.35.0, the "latest" tag is also updated. Tags without the architecture part
-# are also created for the default architecture.
+# Creates and pushes all Docker images aliases for the requested architectures
+# set in the environment variable ALL_REQUESTED_ARCH.  If the value of the
+# global variable TAG_BASE is a version tag such as v0.35.0, the "latest" tag
+# is also updated. Tags without the architecture part are also created for the
+# default architecture.
 # As an example, for amd64 (the default architecture) and the tag v0.35.0, the
 # following tags would be created:
 #  - certbot/certbot:v0.35.0
@@ -30,14 +37,15 @@ source "$WORK_DIR/lib/common"
 # For other tags such as "nightly", aliases are only created for the default
 # architecture where the tag "nightly" would be used without an architecture
 # part.
-# Usage: TagAndPushForAllArch [IMAGE NAME]
+# Usage: TagAndPushForAllRequestedArch [IMAGE NAME]
 #   where [IMAGE NAME] is the name of the Docker image in the Docker repository
 #   such as "certbot" or "dns-cloudflare".
 # Read globals:
 # * TAG_BASE
-TagAndPushForAllArch() {
+# * ALL_REQUESTED_ARCH
+TagAndPushForAllRequestedArch() {
     DOCKER_REPO="${DOCKER_HUB_ORG}/${1}"
-    for TARGET_ARCH in "${ALL_TARGET_ARCH[@]}"; do
+    for TARGET_ARCH in "${ALL_REQUESTED_ARCH[@]}"; do
         docker push "${DOCKER_REPO}:${TARGET_ARCH}-${TAG_BASE}"
 
         if [[ "${TAG_BASE}" =~ ^v[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
@@ -56,9 +64,9 @@ TagAndPushForAllArch() {
 }
 
 # Step 1: Certbot core Docker
-TagAndPushForAllArch "certbot"
+TagAndPushForAllRequestedArch "certbot"
 
 # Step 2: Certbot DNS plugins Docker images
 for plugin in "${CERTBOT_PLUGINS[@]}"; do
-    TagAndPushForAllArch "${plugin}"
+    TagAndPushForAllRequestedArch "${plugin}"
 done