To expand on the answer provided by @gilles, here's one way to create a disk image containing a formatted filesystem by first creating a filesystem (of type ESP in this example) within a file and then assembling that to a valid disk image; no root, mounts or loop devices required:

diskimg=diskimg    # filename of resulting disk image
size=$((260*(1<<20))) # desired size in bytes, 260MB in this case
alignment=1048576  # align to next MB (https://www.thomas-krenn.com/en/wiki/Partition_Alignment)
size=$(( (size + alignment - 1)/alignment * alignment ))  # ceil(size, 1MB)

# mkfs.fat requires size as an (undefined) block-count; seem to be units of 1k
mkfs.fat -C -F32 -n "volname" "${diskimg}".fat $((size >> 10))

# insert the filesystem to a new file at offset 1MB
dd if="${diskimg}".fat of="${diskimg}" conv=sparse obs=512 seek=$((alignment/512))
# extend the file by 1MB
truncate -s "+${alignment}" "${diskimg}"

# apply partitioning
parted --align optimal "${diskimg}" mklabel gpt mkpart ESP "${offset}B" '100%' set 1 boot on

The above approach has the side-benefit of being sparse when used on a filesystem that supports sparse files; the resulting "262MB" file occupies less than 200kB on disk:

du -h --apparent diskimg; du -h diskimg
262M    diskimg
196K    diskimg

Here's a diagram of the resulting image file:

To expand on the answer provided by @gilles, here's one way to create a disk image containing a formatted filesystem by first creating a filesystem (of type ESP in this example) within a file and then assembling that to a valid disk image; no root, mounts or loop devices required:

diskimg=diskimg    # filename of resulting disk image
size=$((260*(1<<20))) # desired size in bytes, 260MB in this case
alignment=1048576  # align to next MB (https://www.thomas-krenn.com/en/wiki/Partition_Alignment)
size=$(( (size + alignment - 1)/alignment * alignment ))  # ceil(size, 1MB)

# mkfs.fat requires size as an (undefined) block-count; seem to be units of 1k
mkfs.fat -C -F32 -n "volname" "${diskimg}".fat $((size >> 10))

# insert the filesystem to a new file at offset 1MB
dd if="${diskimg}".fat of="${diskimg}" conv=sparse obs=512 seek=$((alignment/512))
# extend the file by 1MB
truncate -s "+${alignment}" "${diskimg}"

# apply partitioning
parted --align optimal "${diskimg}" mklabel gpt mkpart ESP "${offset}B" '100%' set 1 boot on

The above approach has the side-benefit of being sparse when used on a filesystem that supports sparse files; the resulting "262MB" file occupies less than 200kB on disk:

du -h --apparent diskimg; du -h diskimg
262M    diskimg
196K    diskimg

For FAT filesystems the Mtools utilities support operating on an offset into a file (ext2/4/etc probably do as well?). This makes it easier, you just create the partitioned image and work on that directly:

diskimg=diskimg
size=$((260*(1<<20))) # desired size in bytes, 260MB in this case
# align to next MB (https://www.thomas-krenn.com/en/wiki/Partition_Alignment)
alignment=1048576

size=$(( (size + alignment - 1)/alignment * alignment ))

# image size is gpt + filesystem size + gpt backup
truncate -s $((size + 2*alignment)) "${diskimg}"

parted --machine --script --align optimal "${diskimg}" mklabel gpt mkpart ESP "${alignment}B" '100%' set 1 boot on

mformat -i "${diskimg}"@@"${alignment}" -t $((size>>20)) -h 64 -s 32 -v "volname"

Here's a diagram of the resulting image file: