feat: update Docker build

Various improvement to Docker build:

• expose container port so that it is clearly stated that port 8080 is exposed
• update alpine packages on build to get up-to-date system
• container is able to run as non-root (more secure)
• use ImageMagick from alpine instead of rebuilding it
• use an up-to-date base image of alpine
• merge multiple shell scripts into a unique shell script
• replace embedded shell script by inline Dockerfile instructions
• use debian source image instead of alpine so that we are closer from a standard install (which is documented for apt-get base distro)

Fix some warnings from linters:

• hadolint warnings from Dockerfile linting
• fix label name that should not contain single quote
• shellcheck warnings from shell scripts linting and switch shell to bash