pgAdmin 4 commit: Update container build to use Alpine Linux and Gunico

Update container build to use Alpine Linux and Gunicorn instead of CentOS and Apache. Fixes #3246

This results in a much more slim-line container, requiring fewer resources to run.
In addition, the majority of the build is now done using the Docker infrastructure, allowing for quicker rebuilds and better use of layers.

Branch
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master

Details
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https://git.postgresql.org/gitweb?p=pgadmin4.git;a=commitdiff;h=05e2e3cb397b21a669129feb067ac44d2fc916dd
Author: Максим Кольцов <kolmax94(at)gmail(dot)com>

Modified Files
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docs/en_US/release_notes_3_0.rst | 1 +
pkg/docker/.dockerignore | 2 +
pkg/docker/Dockerfile | 87 +++++++++++++++++++++-------------------
pkg/docker/README | 58 +++++++++++++--------------
pkg/docker/build.sh | 58 +++++----------------------
pkg/docker/config_distro.py | 4 ++
pkg/docker/entry.sh | 29 --------------
pkg/docker/entrypoint.sh | 26 ++++++++++++
pkg/docker/pgadmin4.conf.j2 | 43 --------------------
pkg/docker/run_pgadmin.py | 4 ++
10 files changed, 120 insertions(+), 192 deletions(-)