Improve Docker build in deployment workflows

[thekaveman]

What this PR does

1. Uses the docker/setup-buildx-action to setup a Docker Buildx builder for the job
2. Uses the docker/build-push-action instead of raw docker commands to build the image
3. Adds layer caching to Docker builds via docker/build-push-action's built-in type=gha for GitHub Actions

Testing the PR

1. Forked over at https://github.com/thekaveman/benefits

2. Set up environment similar to this repository

3. Made a small modification to workflow to skip the actual deploy

4. Merged change (thekaveman/benefits#1) to see workflow run and succeed with new build steps (some output copied below):

   Set up Docker Buildx

   Run docker/setup-buildx-action@v1
   Docker info
   Creating a new builder instance
   Booting builder
   Inspect builder
   BuildKit version
      moby/buildkit:buildx-stable-1 => buildkitd github.com/moby/buildkit v0.9.1 966bcf4aa3ef397a9d6025fade18f2d59a5bf29d

   Build, tag, and push to Amazon ECR

   Run docker/build-push-action@v2
   Docker info
     /usr/bin/docker buildx build --tag ***.dkr.ecr.***.amazonaws.com/cal-itp-benefits-client:6432a9d0ccee93521950e5ab4e42f1deb1a752d8 --iidfile /tmp/docker-build-push-oYNvON/iidfile --metadata-file 
     /tmp/docker-build-push-oYNvON/metadata-file --cache-from type=gha,scope=benefits --cache-to type=gha,scope=benefits,mode=max --builder builder-b74c8782-c697-4769-b727-5c4bf83b40fc --push .
   #2 [internal] load .dockerignore
   #2 transferring context: 171B done
   
   ...
   
   #17 [ 1/11] FROM docker.io/library/python:3.9-slim@sha256:8ca34144574637bb24baa62a767d1ef8be821d9ee7e6a16fa3ab628cb7f96c27
   #17 sha256:36629b83aba2df8771daed1573adb172bd6878a96fd9327e4f32f0174aec1c5c 233B / 233B 0.1s done
   #17 sha256:7338abefe51cf89eefc207e03876d5c2183b5281ceb57a9067b3e13629303bab 2.64MB / 2.64MB 0.1s done
   #17 sha256:6ad2a11ca37b37ebd142aaa2d25901a91cee64d326c76c0133e42d864a680ed5 1.08MB / 1.08MB 0.1s done
   #17 sha256:e9edbe81a0012ed0e8f2e0a8fba50eb92d4c7b8c3d66a2695c6d6c70457d995e 11.02MB / 11.02MB 0.2s done
   #17 sha256:7d63c13d9b9b6ec5f05a2b07daadacaa9c610d01102a662ae9b1d082105f1ffa 18.87MB / 31.36MB 0.2s
   #17 sha256:7d63c13d9b9b6ec5f05a2b07daadacaa9c610d01102a662ae9b1d082105f1ffa 31.36MB / 31.36MB 0.3s
   #17 sha256:7d63c13d9b9b6ec5f05a2b07daadacaa9c610d01102a662ae9b1d082105f1ffa 31.36MB / 31.36MB 0.4s done
   #17 extracting sha256:7d63c13d9b9b6ec5f05a2b07daadacaa9c610d01102a662ae9b1d082105f1ffa
   #17 extracting sha256:7d63c13d9b9b6ec5f05a2b07daadacaa9c610d01102a662ae9b1d082105f1ffa 1.0s done
   #17 extracting sha256:6ad2a11ca37b37ebd142aaa2d25901a91cee64d326c76c0133e42d864a680ed5 0.1s done
   #17 DONE 1.5s
   
   ...
   
   #6 [ 2/11] RUN useradd --create-home --shell /bin/bash calitp &&
     apt-get update &&
     apt-get install -qq --no-install-recommends gettext nginx &&
     mkdir -p /var/cache/nginx &&
     chown -R calitp /var/cache/nginx &&
     mkdir -p /var/lib/nginx &&
     chown -R calitp /var/lib/nginx &&
     mkdir -p /var/log/nginx &&
     chown -R calitp /var/log/nginx &&
     touch /var/run/nginx.pid &&
     chown -R calitp /var/run/nginx.pid &&
     mkdir -p /home/calitp/app/config &&
     mkdir -p /home/calitp/app/run &&
     mkdir -p /home/calitp/app/static &&
     chown -R calitp /home/calitp
   
   #6 0.564 Get:1 http://deb.debian.org/debian bullseye InRelease [116 kB]
   #6 0.575 Get:2 http://security.debian.org/debian-security bullseye-security InRelease [44.1 kB]
   #6 0.598 Get:3 http://deb.debian.org/debian bullseye-updates InRelease [39.4 kB]
   #6 0.691 Get:4 http://security.debian.org/debian-security bullseye-security/main amd64 Packages [48.0 kB]
   #6 0.781 Get:5 http://deb.debian.org/debian bullseye/main amd64 Packages [8180 kB]
   #6 0.887 Get:6 http://deb.debian.org/debian bullseye-updates/main amd64 Packages [2300 B]
   #6 1.825 Fetched 8430 kB in 1s (6525 kB/s)
   
   ...

5. Pushed one more small change to trigger the workflow one more time with layer caching! (some output copied below):

   Build, tag, and push to Amazon ECR

   #17 [ 1/11] FROM docker.io/library/python:3.9-slim@sha256:8ca34144574637bb24baa62a767d1ef8be821d9ee7e6a16fa3ab628cb7f96c27
   #17 resolve docker.io/library/python:3.9-slim@sha256:8ca34144574637bb24baa62a767d1ef8be821d9ee7e6a16fa3ab628cb7f96c27 done
   #17 DONE 0.0s
   
   #5 importing cache manifest from gha:10117132071653542490
   #5 DONE 0.5s
   
   #8 [internal] load build context
   #8 transferring context: 1.02MB 0.0s done
   #8 DONE 0.0s
   
   #6 [ 2/11] RUN useradd --create-home --shell /bin/bash calitp &&
     apt-get update &&
     apt-get install -qq --no-install-recommends gettext nginx &&
     mkdir -p /var/cache/nginx &&
     chown -R calitp /var/cache/nginx &&
     mkdir -p /var/lib/nginx &&
     chown -R calitp /var/lib/nginx &&
     mkdir -p /var/log/nginx &&
     chown -R calitp /var/log/nginx &&
     touch /var/run/nginx.pid &&
     chown -R calitp /var/run/nginx.pid &&
     mkdir -p /home/calitp/app/config &&
     mkdir -p /home/calitp/app/run &&
     mkdir -p /home/calitp/app/static &&
     chown -R calitp /home/calitp
   #6 CACHED
   
   #7 [ 3/11] WORKDIR /home/calitp/app
   #7 CACHED
   
   #9 [ 4/11] COPY requirements.txt requirements.txt
   #9 CACHED
   
   #10 [ 5/11] RUN pip install --no-cache-dir -r requirements.txt
   #10 CACHED
   
   ...

More information

Should get us closer to #133

Closes #134

EDIT: Workflow run after this PR was merged showing new build process