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/lib/common

@@ -31,3 +31,23 @@ export CERTBOT_PLUGINS=(
     "dns-linode"
     "dns-sakuracloud"
 )
+
+# Parses the requested architecture string and sets ALL_REQUESTED_ARCH to
+# result.
+# Usage: ParseRequestedArch [all|amd64|arm32v6|arm64v8]
+ParseRequestedArch() {
+    REQUESTED_ARCH="${1}"
+    if [[ "${REQUESTED_ARCH}" == "all" ]]; then
+        ALL_REQUESTED_ARCH=("${ALL_TARGET_ARCH[@]}")
+        return 0
+    fi
+    for TARGET_ARCH in "${ALL_TARGET_ARCH[@]}"; do
+        if [[ "${TARGET_ARCH}" == "${REQUESTED_ARCH}" ]]; then
+            ALL_REQUESTED_ARCH=("${REQUESTED_ARCH}")
+            return 0
+        fi
+    done
+    # If we didn't return above, REQUESTED_ARCH has an unexpected value.
+    echo "Unexpected target architecture \"${REQUESTED_ARCH}\"". >&2
+    exit 1
+}