Hello,
Sorry to insist, but could anyone help me understand the problem I'm having
in using Emacs with standalone context ?
The following file compiles perfectly from a terminal :

/home/aragorn/texlive/2018/bin/x86_64-linux/context  test.tex

/home/aragorn/context/tex/texmf-linux-64/bin/context  test.tex (latest beta)

With Emacs, it compiles perfectly if you use the 2018 texlive context
binary.

If I want to use the latest beta context binary, I need to add this to my
~/.emacs file :

(setenv "PATH" "/home/aragorn/context/tex/texmf-linux-64/bin/:$PATH" t)

And there, I encounter a compilation problem :

Running `ConTeXt Full' on `pouf' with ``context --purgeall --nonstop
pouf.tex''resolvers       | trees | analyzing 'home:texmf'
mtx-context     | run 1: luatex
--fmt="/home/aragorn/context/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luatex/cont-en"
--interaction="nonstopmode" --jobname="pouf"
--lua="/home/aragorn/context/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luatex/cont-en.lui"
--no-parse-first-line --c:currentrun=1 --c:fulljobname="./pouf.tex"
--c:input="./pouf.tex" --c:kindofrun=1 --c:maxnofruns=9 --c:nonstopmode
--c:purgeall "cont-yes.mkiv"

This is LuaTeX, Version 1.08.0 (TeX Live 2018)
 system commands enabled.

resolvers       > trees > analyzing 'home:texmf'
open source     > level 1, order 1, name
'/home/aragorn/context/tex/texmf-context/tex/context/base/mkiv/cont-yes.mkiv'
job             > passes > version mismatch: 1.3 <> 1.31
system          >
system          > ConTeXt  ver: 2018.08.20 17:47 MKIV beta  fmt: 2018.8.20
int: english/english
system          >
system          > 'cont-new.mkiv' loaded
open source     > level 2, order 2, name
'/home/aragorn/context/tex/texmf-context/tex/context/base/mkiv/cont-new.mkiv'
system          > beware: some patches loaded from cont-new.mkiv
close source    > level 2, order 2, name
'/home/aragorn/context/tex/texmf-context/tex/context/base/mkiv/cont-new.mkiv'

system          > files > jobname 'pouf', input './pouf', result 'pouf'
fonts           > latin modern fonts are not preloaded
languages       > language 'en' is active
open source     > level 2, order 3, name '/home/aragorn/pouf.tex'
fonts           > preloading latin modern fonts (second stage)
fonts           > 'fallback modern-designsize rm 12pt' is loaded
structure       > sectioning > chapter @ level 2 : 0.1 -> First chapter
metapost        > initializing instance 'metafun:1' using format 'metafun'
and method 'default'
metapost        > loading 'metafun' as
'/home/aragorn/context/tex/texmf-context/metapost/context/base/mpiv/metafun.mpiv'
using method 'default'
metapost        > initializing number mode 'scaled'

lua error       > lua error on line 10 in file /home/aragorn/pouf.tex:

...ext/tex/texmf-context/tex/context/base/mkiv/node-nut.lua:190: attempt to
call a nil value (global 'getid')
stack traceback:
    ...ext/tex/texmf-context/tex/context/base/mkiv/node-nut.lua:190: in
upvalue 'getexpansion'
    ...ext/tex/texmf-context/tex/context/base/mkiv/font-mps.lua:343: in
function <...ext/tex/texmf-context/tex/context/base/mkiv/font-mps.lua:338>
    ...ext/tex/texmf-context/tex/context/base/mkiv/font-mps.lua:437: in
local 'boxtomp'
    ...ext/tex/texmf-context/tex/context/base/mkiv/mlib-pps.lua:1894: in
function <...ext/tex/texmf-context/tex/context/base/mkiv/mlib-pps.lua:1891>
    (...tail calls...)

 1
 2     \startusableMPgraphic{NumberHead}
 3          draw outlinetext.f
 4              ("\bf\namedheadnumber{chapter}")
 5              (withcolor "lightgray")
 6               ysized 50pt ;
 7     \stopusableMPgraphic
 8
 9     \unexpanded\def\processMPheadnumber#1%
10 >>    {\useMPgraphic{NumberHead}}
11
12     \setuphead
13        [chapter]
14        [command=\HeadTitle,
15         headstyle=\ss,
16         numbercommand=\processMPheadnumber,
17        ]
18
19
20     \unexpanded\def\HeadTitle#1#2%


open source     > level 3, order 4, name
'/home/aragorn/context/tex/texmf-context/tex/context/sample/common/ward.tex'
close source    > level 3, order 4, name
'/home/aragorn/context/tex/texmf-context/tex/context/sample/common/ward.tex'
backend         > xmp > using file
'/home/aragorn/context/tex/texmf-context/tex/context/base/mkiv/lpdf-pdx.xml'
pages           > flushing realpage 1, userpage 1, subpage 1
close source    > level 2, order 4, name '/home/aragorn/pouf.tex'
close source    > level 1, order 4, name
'/home/aragorn/context/tex/texmf-context/tex/context/base/mkiv/cont-yes.mkiv'

mkiv lua stats  > used config file: selfautoparent:/texmf/web2c/texmfcnf.lua
mkiv lua stats  > used cache path:
/home/aragorn/context/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e
mkiv lua stats  > resource resolver: loadtime 0.007 seconds, 1 scans with
scantime 0.000 seconds, 0 shared scans, 31 found files, scanned paths:
/home/aragorn/texmf
mkiv lua stats  > stored bytecode data: 437 modules (0.226 sec), 93 tables
(0.015 sec), 530 chunks (0.241 sec)
mkiv lua stats  > traced context: maxstack: 1328, freed: 5, unreachable:
1323
mkiv lua stats  > cleaned up reserved nodes: 58 nodes, 9 lists of 455
mkiv lua stats  > node memory usage: 2 glue, 2 kern, 2 penalty, 14
attribute, 50 glue_spec, 4 attribute_list, 2 temp, 2 user_defined
mkiv lua stats  > node list callback tasks: 8 unique task lists, 6
instances (re)created, 55 calls
mkiv lua stats  > used backend: pdf (backend for directly generating pdf
output)mkiv lua stats  > jobdata time: 0.001 seconds saving, 0.000 seconds
loading
mkiv lua stats  > callbacks: internal: 179, file: 180, direct: 2, late: 1,
function 529, total: 891
mkiv lua stats  > loaded patterns: en::2, load time: 0.000
mkiv lua stats  > result saved in file: pouf.pdf, compresslevel 3,
objectcompresslevel 3
mkiv lua stats  > loaded fonts: 4 files: latinmodern-math.otf,
lmroman10-bold.otf, lmroman10-regular.otf, lmroman12-regular.otf
mkiv lua stats  > font engine: otf 3.104, afm 1.513, tfm 1.000, 9
instances, 3 shared in backend, 3 common vectors, 0 common hashes, load
time 0.238 seconds
mkiv lua stats  > metapost: 0.001 seconds, loading: 0.018, execution:
0.000, n: 1, average: 0.019, instances: 1, luacalls: 7, memory: 2.608 M
mkiv lua stats  > used platform: linux-64, type: unix, binary subtree:
texmf-linux-64
mkiv lua stats  > used engine: luatex version 1.08 with functionality level
6731, banner: this is luatex, version 1.08.0 (tex live 2018)
mkiv lua stats  > control sequences: 46533 of 65536 + 100000
mkiv lua stats  > lua properties: engine: lua 5.3, used memory: 61 MB (ctx:
61 MB), hash type: lua, hash chars: min(64,40), symbol mask: utf (τεχ)
mkiv lua stats  > runtime: 0.613 seconds, 1 processed pages, 1 shipped
pages, 1.631 pages/second
mtx-context     | fatal error: return code: 1

TeX Output exited abnormally with code 1 at Wed Aug 22 13:01:54

########
test.tex
########

\startusableMPgraphic{NumberHead}
     draw outlinetext.f
         ("\bf\namedheadnumber{chapter}")
         (withcolor "lightgray")
          ysized 50pt ;
\stopusableMPgraphic

\unexpanded\def\processMPheadnumber#1%
  {\useMPgraphic{NumberHead}}

\setuphead
   [chapter]
   [command=\HeadTitle,
    headstyle=\ss,
    numbercommand=\processMPheadnumber,
   ]


\unexpanded\def\HeadTitle#1#2%
  {\framed
    [frame=off,
     bottomframe=on,
     width=broad,
     align={broad,nothyphenated,left}]
    {#1\blank[nowhite]#2}}

  \starttext
  \startchapter[title=First chapter]
    \input ward
  \stopchapter
  \stoptext

Thanks
Fabrice