Description of problem:
Image builder does not support the use of a dot inside a username, whereas it seems to be valid as per IEEE Std 1003.1-2001 which allows letters, digits, underscores, periods, at signs (@) and dashes.

Version-Release number of selected component (if applicable):
osbuild-53-2.el8
osbuild-composer-46.3-1.el8_6

How reproducible:
Always

Steps to Reproduce:
1. Push the below blueprint

name = "dotuser"
description = "reproducer user with a dot"
version = "0.0.1"
modules = []
groups = []
distro = ""

[[customizations.user]]
name = "foo.bar"
password = "$6$R4EexOTlkvDDwsK8$WkL4pU.JGHSO9LmpI/SMKYbunZU.t7BYLsZs1B9m0SeOH4RBICGlbScNRl0jUri3CBwasAd/sdZMkDp4SGoBq."

2. Start a compose


Actual results:
osbuild-composer fails to build the image very quickly (during some pre-checks I guess). The compose log is empty and the composer does not show any relevant error.

Jul 06 12:57:43 localhost.localdomain osbuild-composer[65518]: time="2022-07-06T12:57:43+02:00" level=info msg="Removing unresponsive job: 7e30f983-5de4-478b-ba9f-1531ad831845\n" func="github.com/osbuild/osbuild-composer/internal/worker.(*Server).WatchHeartbeats" file="/builddir/build/BUILD/osbuild-composer-46.3/_build/src/github.com/osbuild/osbuild-composer/internal/worker/server.go:87"
Jul 06 12:57:43 localhost.localdomain osbuild-composer[65518]: time="2022-07-06T12:57:43+02:00" level=error msg="Error finishing unresponsive job: error finishing job: job ws canceled" func="github.com/osbuild/osbuild-composer/internal/worker.(*Server).WatchHeartbeats" file="/builddir/build/BUILD/osbuild-composer-46.3/_build/src/github.com/osbuild/osbuild-composer/internal/worker/server.go:90"

Additional info:
- stracing the composer shows it does not report properly the error causing the issue:

27710 09:24:44.445774 write(1<pipe:[138414]>, "{\"type\": \"https://osbuild.org/validation-error\", \"title\": \"JSON Schema validation failed\", \"success\": false, \"errors\": [{\"message\": \"'foo.bar' does not match any of the regexes: '^[A-Za-z0-9_][A-Za-z0-9_-]{0,31}$'\", \"path\": [\"pipelines\", 1, \"stages\", 6, \"options\", \"users\"]}]}\n", 277) = 277 <0.000041>

- the regex in question comes from the JSON schema of the org.osbuild.users stage (/usr/lib/osbuild/stages/org.osbuild.users). Modifying the line 29 by inserting \\\. into the regular expression in order to allow the dot inside a username works around the issue:

	 21 SCHEMA = """
	 22 "additionalProperties": false,
	 23 "properties": {
	 24   "users": {
	 25     "additionalProperties": false,
	 26     "type": "object",
	 27     "description": "Keys are usernames, values are objects giving user info.",
	 28     "patternProperties": {
	 29       "^[A-Za-z0-9_][\\\.A-Za-z0-9_-]{0,31}$": {
	 30         "type": "object",
	 31         "properties": {

- note that I didn't check this updated regex is correct and still prevents an invalid name.