Move Julia post create commands into the image (#650)

containers/julia/.devcontainer/devcontainer.json

@@ -3,6 +3,6 @@
 	"name": "Julia (Community)",
 	"image": "ghcr.io/julia-vscode/julia-devcontainer",
 	"extensions": ["julialang.language-julia"],
-	"postCreateCommand": "julia --project=. -e 'using Pkg; if (isfile(joinpath(pwd(),\"Manifest.toml\")) || isfile(joinpath(pwd(),\"JuliaManifest.toml\"))) && (isfile(joinpath(pwd(),\"Project.toml\")) || isfile(joinpath(pwd(),\"JuliaProject.toml\"))); Pkg.instantiate(); end'",
+	"postCreateCommand": "/julia-devcontainer-scripts/postcreate.jl",
 	"remoteUser": "vscode"
 }